Aggression
Wayne A. Warburton
Macquarie University
Craig A. Anderson
Iowa State University
Warburton, W. A., & Anderson, C. A. (in press). Aggression. In V. Zeigler-Hill & T. K.
Shackelford (Eds.), The SAGE Handbook of Personality and Individual Differences.
Wayne Warburton
Department of Psychology
Macquarie University
NSW, 2109, Australia
Phone: +61 2 9850 8643
Fax: +61 2 9850 8062
Email: [email protected]
Craig A. Anderson
Department of Psychology,
Iowa State University
W112 Lagomarcino Hall, Ames, IA 50011-3180
Phone: +1 515 294 3118
Fax: +1 515 294 6424
Email: [email protected]
“If it's natural to kill, how come men have to go into training to learn how?”
Joan Baez
Every history of humanity is peppered with incidents related to human aggression
– war, conflict, interpersonal violence, and everyday hurts and harms. Although the
contexts range widely, societal aggression and violence are ultimately driven by the
intent of one person to hurt another, with some people being much more predisposed to
aggression than others. For this reason, understanding within-person factors that increase
an individual’s likelihood of aggression is crucial to identifying the causes of aggressive
behavior as well as the strategies that have the best chance of moderating it. This chapter
examines aggressive behavior through the lens of a current model of aggression – the
General Aggression Model (GAM; Anderson & Bushman, 2002; Gilbert, Daffern, &
Anderson, in press; Warburton & Anderson, in press) - that emphasizes the way in which
an individual’s characteristics interact with situational cues and triggers from the
environment to produce aggressive behavior.
Central to the GAM is prior research about factors within a person that predispose
them to aggression: individual differences in traits, genetics, biology, and learned
experience that make one person more likely than another to behave aggressively.
Because the GAM is a model of the processes that lead to an individual behaving
aggressively in the moment, it also describes the underlying biological, neurocognitive,
social and other psychological factors and processes that may influence an episode of
aggression. It also incorporates outcomes of the aggressive event, that is, the learnings
that feed back into the person’s stable psychological makeup, learnings that change the
person's expectations and beliefs about how well or poorly future aggressive (or
nonaggressive) behaviors will work. Of course, individual differences in the
predisposition to aggress can never fully explain aggressive behavior – environmental
factors that trigger aggressive tendencies are also important factors in the aggression
equation, and are prominent in the GAM as well.
We begin this chapter by characterizing aggression, by describing historical and
modern theories of aggressive behavior, and then by explaining the GAM, which draws
together many of these theories. Key research methodologies and findings relevant to
aggressive behavior are outlined, along with issues around the development of aggressive
behavior and the challenges involved in treating the aggressive individual. In terms of the
wider social implications of aggression research, we argue for a truly evidence-based
approach to managing societal aggression and note directions for future research.
1. Characteristics of aggression
Definitions
Defining aggression is far from straightforward because there are several key
issues that need to be taken into account. Historically, it has been difficult to compare
research findings and theories related to aggressive behavior because aggression has been
defined and operationalized differently both within and between scientific disciplines.
More recently, aggression researchers from psychology have converged on an approach
that is broadly inclusive of a range of behaviors that involve one person harming another
and is exemplified in the widely-used definition of Anderson and Bushman (2002):
aggression is any behavior enacted with the intention to harm another person who is
motivated to avoid that harm. This definition is broad enough to capture a wide range of
aggressive behaviors, while also excluding activities that can “hurt” a target person but to
which the target of the hurt willingly consents (e.g., undergoing surgery).
Another key issue is that many people, including some misinformed professionals,
use the term aggression interchangeably with related but conceptually distinct phenomena
including anger (which is a feeling, not a behavior), hostility (an internal state
incorporating various hostile thoughts and feelings but which is not a behavior) and being
competitive (related to one’s desire to compete and succeed rather than to harm someone).
These three phenomena all relate to internal states on which individuals may differ (e.g.,
trait anger, hostile attributional bias, highly competitive), and that may predispose people
to behave aggressively, however none involve aggressive behavior per se. There is no
question that in the field of psychology, aggression refers only to a behavior, and not to a
mindset or an emotional state. Being angry, holding attitudes in which one wishes the
worst for others, and being motivated to win and succeed, can all increase the likelihood
that someone will behave aggressively, but they often do not lead to aggressive behavior.
Therefore, such terms cannot be meaningfully exchanged for the term aggression and
should be clearly differentiated.
A final issue involves a similar problem - using the terms “violence” and
“aggression” interchangeably. In psychology, violence is a subtype of aggression, usually
differentiated from “everyday” aggression by the degree of harm or potential harm to
victims. Violence is typically conceptualized as aggressive behavior that has a high
potential of causing harm extreme enough to require medical attention or to cause death
(Warburton & Anderson, 2015). We emphasize potential because a failed attempt to kill
someone is still regarded as a violent behavior. Shooting at someone in an attempt to kill
them is a violent act, even when unsuccessful. Because the developmental and mental
health impact of emotional harm can be very severe, many psychologists extend this
definition of violence to include causing severe emotional harm (such as in the case of
domestic violence and child abuse; see Warburton & Anderson, in press). Thus, all
violent behavior is aggression because it is undertaken with the intent to harm another,
but most aggression is not violence because many aggressive behaviors do not have the
potential to inflict extreme harm. It should be noted that this definition of violence is not
the same as that used when referring to “violent crime,” which is a legal term rather than
a scientific term. When the terms violence, aggression, and violent crime are treated as
synonymous there is considerable potential for confusion and miscommunication
between researchers, policy-makers, professionals, and the general public, most notably
when research on aggressive behavior is portrayed as meaning "societal violence."
Because aggressive behavior is conceived as existing on a continuum from fairly mild to
extreme, and because the underlying psychological processes and risk factors are largely
the same regardless of extremity, research on milder forms of aggression is relevant to
understanding violence. Nonetheless, it is important to keep in mind that differences in
the extremity of aggressive acts are likely to involve differences in the number and
extremity of the specific risk and protective factors involved. Thus, it is important (as in
all fields of scientific research) to be aware of the need to be cautious when attempting to
generalize from one form of aggression to another, be it from mild to severe, severe to
mild, or even among different types of aggression at the same level of severity.
Types and Characteristics of Aggression
Humans can hurt each other in a wide variety of ways and so it is helpful to
characterize the different forms that aggression can take and the different functions it
serves. Physical aggression involves physically harming another (e.g., punching,
slapping, hitting, biting, kicking, clubbing, stabbing, shooting) whereas verbal aggression
includes the many ways that one person can harm another with spoken words (e.g., verbal
abuse, screaming, swearing, name calling). Relational aggression involves hurting
another person’s reputation, friendships, or other interpersonal relationships through what
is said to others verbally or digitally. Digital aggression – including cyberbullying – is a
more recent phenomenon that includes various forms of harming another through digital
and online devices (e.g., hate blogs, hurtful text messages, damaging social media posts).
In addition, aggression can be direct (the victim is physically present when they are being
harmed); indirect (the victim is harmed in their absence); or displaced (the aggressor
harms an innocent, more vulnerable target because he or she is unable to harm the true
target of the aggressive impulse).
Aggression also differs by function. For example, aggression may simply function
to harm a person who has caused upset or anger through a provocation. This type of
response is often characterized as a form of reactive, hostile aggression (also called
affective, angry, hot, impulsive, or retaliatory aggression). Alternatively, aggression may
involve a cold and calculated plan to hurt another in order to gain something of value (i.e.,
instrumental, pro-active, planned, or cold aggression). This hostile-instrumental
aggression dichotomy (under a variety of labels) is one of the most studied phenomena in
the field of aggressive behavior. Some forms of aggression, however, are not a reaction to
a particular provocation or the result of a clearly articulated plan. Rather, they may
involve an automatic aggressive response that might be driven by a hard-wired self-
protection mechanism (e.g., fight or flight) or may involve a script for aggressive
behavior that is so commonly carried out that the response is no longer a thoughtful one
(although earlier in the aggressor's life—prior to the script becoming automatized—it
may have required significant cognitive effort).
Of course, trying to fit an act of aggression into a function category can be
problematic. It is not hard to think of instances of aggression that are an exception. For
example, what about the school shooter whose rage at years of social rejection drives
them to carefully plan a mass shooting? Such instances don't fit the traditional hostileinstrumental dichotomy and can only be characterized if one broadens the way that
functions of aggression are conceptualized (e.g., Bushman & Anderson, 2001). More
recently, some aggression theorists have suggested that rather than trying to characterize
each act of aggression as either hostile or instrumental, it may be more useful note that
much aggression has multiple underpinnings. To this end, aggressive acts may be better
located on three key dimensions: the degree to which the aggressor's goal is to harm the
victim versus benefit the perpetrator, the level of hostile or agitated emotion that is
present, and the degree to which the aggressive act was thought-through (Anderson &
Huesmann, 2003; Warburton & Anderson, 2015).
2. Theories of aggression
Because the focus of this volume and this chapter is on personality and individual
differences, it is important here to be clear that the authors of this chapter view stable
personality as including a wide range of factors that display some consistency across time
and situations, including traits, temperament, attitudes, beliefs, schemata, scripts for
behavior, and other knowledge structures (e.g., Bandura, 1986; Mischel & Shoda, 1995).
That is, personality is conceived as including characteristics that are both genetically
inherited and learned with experience. Further, knowledge structures are viewed as
playing an important role in influencing what situations a person will seek out or avoid,
thus adding to the stability of existing schemata and scripts.
Using this broad definition, many theories of aggression have a clear component
related to personality and/or individual differences. Theories that emphasize traits,
aggressive drives, and learned aggressive behavior can be characterized as highlighting
personality factors. Other theories emphasize individual differences in the influence of
genetics, hormones, malformed or damaged brain structures, and levels of cortical and
nervous system arousal. Even theories rooted in social psychology tend to focus on the
interaction between within-person factors and cues from the environment, thus
incorporating personality/individual difference factors within their models. As will be
seen in the following models and theories of aggression, the assumption that each
aggressor brings their own characteristics to an act of aggression is ubiquitous. However,
the expression of any “person factor”, as with epigenetics, is influenced by the
environment in which it occurs.
Early Theories of Aggression
Aggression as a drive. Some of the earliest theories concerning aggression in
psychology characterized it as stemming from an aggressive instinct or drive. Freud,
post-World War I (e.g., Freud, 1920), added to his earlier drive model by positing that
drives toward pleasure and an instinct to live (e.g., the libido) were countered by "death
drives" (Todestriebe) through which the individual seeks to die in order to eliminate
internal tension. In these later formulations, Freud saw the libido as functioning to render
innocuous the “destroying instinct” (sometimes referred to as the Thanatos), by focusing
it largely outwards, as aggression. That is, the price of deflecting self-destruction is the
harming of others. Freud also thought that, like the libido, aggressive drives could be
sublimated (channeled into constructive pursuits), but concluded late in his career that
they could not be eliminated altogether.
This characterization of aggression as inborn, constantly seeking an outlet, and
able to be diverted, has persisted in a range of theories put forward since Freud. Later
psychoanalysts (e.g., Anna Freud, Hartmann, & Kris, 1949) suggested that aggressive
urges are somatically rooted and that aggressive energy is constantly generated. This urge
may lead the person to harm themselves or others unless it can be sublimated into a more
socially acceptable activity (e.g., a socially appropriate physical activity).
Animal behaviorists such as Konrad Lorenz (1966) have produced similar models
based on these “hydraulic” principles. Lorenz suggested that in both animals and humans
an excitation occurs in instinctual centers in the nervous system, causing a “pent up”
energy that needs to be released. In terms of aggression, if the organism does not
regularly encounter stimuli that release their aggressive energy, then aggressive behavior
will occur spontaneously, much in the way a pressure cooker explodes if steam is not
released. For Lorenz, this build-up of aggressive energy occurs regardless of external
circumstances, and cannot be lessened by changing a person’s lot in life, but can be
sublimated into non-aggressive activities that discharge aggressive energy.
It should be noted here that the notion of cathartic release of aggressive energy is
not supported by empirical findings (see Geen & Quanty, 1977, for a review). Both
longitudinal and experimental studies show that behaving aggressively increases the
likelihood of subsequent aggression rather than decreasing it (e.g., Bushman, Baumeister,
& Stack, 1999).
The frustration-aggression hypothesis. Partially in response to the build-up to
World War II, and taking into account the spreading influence of psychodynamic theories
in the US, Dollard, Doob, Miller, Mowrer and Sears (1939) proposed the first modern
systematic theory of aggression. Building on Freud’s early notion that aggression
occurred when people were thwarted in their pursuit of pleasure (e.g., when their libido
was frustrated), Dollard and colleagues focused on the frustration caused when a goal is
blocked. In their seminal 1939 paper they suggested that “the occurrence of aggressive
behavior always presupposes the existence of frustration” and that “the existence of
frustration always leads to some form of aggression” (p. 1). Dollard and colleagues
provided some empirical support for this view, but this approach did not stand up to close
scrutiny. Clearly, frustration does not inevitably lead to aggression and not every act of
aggression can be traced back to frustration. In 1941, Miller (with the collaboration of his
Yale colleagues) published a paper clarifying the theory and raising the possibility that
frustration can lead to responses other than aggression (e.g., finding another way to
achieve a thwarted goal) and suggesting that people may learn though their experiences
to respond to frustrations with a range of responses, which may be aggressive or nonaggressive.
Learning theories. All three types of major learning theory (i.e., classical
conditioning, operant conditioning, and social (observational) learning) have been
applied to the study of aggressive behavior. The earliest of these is the classical
conditioning approach pioneered by Pavlov. The cornerstone of this approach is the basic
truth that when humans (and many animals) experience two things together they learn to
associate these two things and start to expect them to occur together. Later work suggests
that the neural representations of these things become “wired together” in the person’s
neural network, so that activating one concept spreads activation to the other (e.g.,
Collins & Loftus, 1977). In this way, if something often co-occurs with aggression, it can
become a cue for aggression. The fact that organisms as simple as cockroaches and
flatworms show such effects demonstrates that learned "expectations" need not be
conscious.
Classical conditioning theory was extended by later theorists such as Thorndike
and Skinner, who showed that people are more likely to repeat a behavior that has been
rewarded and less likely to repeat a behavior that has been punished. Through these
operant conditioning processes, some behaviors become more prevalent in a person’s (or
animal's) behavioral repertoire. Research shows that when aggressive behavior is
rewarded (positively reinforced) or results in the removal of something aversive
(negatively reinforced), the person is more likely to behave aggressively in the future.
Indeed, even when aggressive behavior has no discernable consequence future aggression
is typically more likely. It is only when aggression has clearly negative consequences that
the likelihood of subsequent aggression is usually reduced. Children learn to discriminate
between situations where aggression has a rewarding or punishing outcome, but this does
not stop conditioned behaviors from generalizing to other situations. The evidence for the
learning of aggression by classical and operant conditioning is strong (e.g., Eron, Walder,
& Lefkowitz, 1971), but such processes cannot explain all instances of acquired
aggression.
In the 1960s researchers such as Bandura showed that children could acquire
aggressive behavior in the absence of classical or operant conditioning, by simply
observing and imitating others. In their classic experiments, children who observed either
a live adult or a film of an actor hitting a “Bobo Doll” in several novel ways tended to
later imitate these behaviors (Bandura, Ross & Ross, 1961; 1963). These findings
informed Bandura’s social learning theory (later called social cognitive theory), which
hypothesized a range of factors and processes that influence how social behaviors (such
as aggression) are learned and enacted (see Nielson, this volume). These include pure
imitation, vicarious learning (learning from the experiences of others rather than by direct
experience), self-efficacy beliefs (beliefs about how capable one is of carrying out a
planned social behavior), outcome beliefs (expectations about the likely outcome of
emitting a particular behavior in a particular situation), and beliefs about what behaviors
are socially normal (Bandura, 1986). Considerable research supports the notion of social
cognitive processes in aggression, and shows that individuals sometimes imitate
aggressive models, particularly if the aggressive model is rewarded for the aggression
and/or is heroic, admired, high status, attractive, or similar.
Arousal: Cognitive labeling and excitation transfer. The 1960s and 1970s saw
the emergence of research that examined the impact of physiological arousal on
aggression and the role of cognitive labels in this process. Research showed that
individuals tend to be more aggressive when physiologically aroused, likely because
arousal from any source will tend to strengthen the dominant action tendency, including
aggressive tendencies (e.g., Geen & O’Neal, 1969). Further, the cognitive labels that an
individual applies to elevated levels of physiological arousal may have an impact on their
subsequent response. For example, studies by Schacter and colleagues (e.g., Schacter &
Singer, 1962) found that when people cognitively labelled physiological arousal as being
due to feelings of anger, they actually became more angry.
Zillmann (1979) extended the concept of cognitive labelling into the field of
aggression with excitation-transfer theory (ETT), which builds on the fact that
physiological arousal, whatever the source, tends to be slow to dissipate. Zillmann
suggested that if two arousing events are separated by only a short amount of time,
arousal from the first event will add to arousal from the second, with the attribution for
the cause of second event then being assumed to account for all of the arousal
experienced. This may produce an inappropriately strong response to the cause of the
second arousing event. For example, if two people are sexually aroused but then become
angry with each other, they may assume their high level of arousal is entirely related to
their angry feelings, come to believe they are very angry, and then produce a behaviorally
aggressive response that is disproportionate to the issue that caused the conflict.
Cognitive theories of aggression
Information processing and script theories. In the late 1970s and the 1980s, the
computer revolution combined with the rapidly growing field of cognitive psychology to
underpin several new ways of thinking about aggressive behavior. Theorists started to
make comparisons between the brain and a computer and social cognitions were
examined from an information processing perspective, with several influential theories of
aggressive behavior emerging. The Social Information Processing (SIP) theory of Dodge
(1986; see also Crick & Dodge, 1994) emphasized the cognitive processes that people use
when interpreting social information: what is perceived and encoded, what attributions
are made, and what action tendencies are considered and ultimately enacted. In SIP
theory a key factor in habitually aggressive children (and presumedly adolescents and
adults) is a hostile attributional bias – a tendency to interpret ambiguous events (such as
being bumped by another person) as being motivated by hostile intent.
Rowell Huesmann’s (1988, 1998) Script Theory of aggression emphasizes the
ways in which people acquire “scripts” for behavior through either direct experience or
observational learning. Encoded in semantic memory, these knowledge structures are
much like the scripts one encounters in a movie or television series, with the individual
having a sense of what role they play in various situations. In any given situation relevant
scripts will be activated. The individual then assesses these scripts for appropriateness in
terms of likely outcomes, social norms, and their own efficacy to carry them out. The
individual then behaves in accordance with their chosen script. Crucially, aggressive
scripts can become chronically accessible (more easily brought to mind and more likely
to be acted on) if an individual habitually responds to particular cues (such as certain
provocations) using that script. The more an aggressive script is acted on, the more it can
generalize to other situations, and the more likely it is to become the automatic, default
response in those situations. Interestingly, much theorizing about and empirical testing of
social scripts derives from artificial intelligence work originally intended to model how
people understand stories (see Schank & Abelson, 1977, for a review of script theory; see
Anderson, 1983, for an empirical application of script theory).
Cognitive Neoassociation Theory. The Cognitive Neoassociation Theory (CNA)
of Berkowitz (1989) also takes an information processing approach, but was the first to
put forward a comprehensive theory of aggressive behavior based on emerging
knowledge about neural processes such as spreading activation. Berkowitz worked on the
base assumption that concepts, emotions, memories, and action tendencies are
interconnected within the brain’s associative neural network. Reworking the frustrationaggression hypothesis, Berkowitz postulated that aversive events (e.g., provocations,
frustrations, unpleasant environments) activate negative affect in the person. These are
linked in the person’s associative neural network to primitive fight and flight tendencies
that are themselves linked to a range of thoughts, feelings and behavioral tendencies.
Both fight and flight mechanisms become activated, but one will come to dominate,
depending on the characteristics of the person and the nature of the situation. When the
dominant response is to “fight”, the person develops a rudimentary anger along with
other thoughts and feelings, and is more likely to be ultimately aggressive. The degree of
likelihood depends, again, on the nature of the activated thoughts and feelings, and on the
situation. In addition, higher order processes (e.g., considering consequences and social
norms, making attributions about the other’s motivation) can also impact on the person’s
final response.
Evolutionary accounts of aggression
Although accounts of aggression from an evolutionary psychology standpoint
have been put forward for over a century, including early theories regarding an
aggressive instinct, it is over the last 2-3 decades that more detailed and reasoned
evolutionary explanations have been put forward. These emphasize the role of aggression
in survival and reproductive success, particularly in terms of aggression facilitating
dominance, territorial gain/maintenance, and mate attraction/retention at an acceptable
physical cost. Evolutionary approaches also tend to explain gender differences in
aggression in terms of the adaptiveness of intra-sexual competition and sexual jealousy.
One notable exemplar is the approach taken by Buss and Shackleford (1997), who
suggest seven adaptive problems for which aggression might have evolved as a solution:
“co-opting the resources of others, defending against attack, inflicting costs on same-sex
rivals, negotiating status and power hierarchies, deterring rivals from future aggression,
deterring mates from sexual infidelity, and reducing resources expended on genetically
unrelated children” (p. 605). One key aspect of all modern evolutionary approaches is the
concept of inclusive fitness, the idea that genes that one shares with others (kin, one's
group) are important, and that therefore the survival of the group is important (see Sela,
this volume, on evolutionary approaches to personality).
Biological theories of aggression
Biological accounts of aggression stress the roles of genetics, hormones,
neurotransmitters, and brain activity (see also Sellers, this volume, and Pickering, this
volume). Some extrapolate from animal research to explain aggression in primates. For
example, the “Motivational Systems Model” (Adams, 2006) explains aggression in terms
of brain mechanisms for defense, offense, and submission. Most biological theories about
humans seek to explain the sorts of persistent antisocial behavior (including aggression
and violence) found in those high in psychopathy, antisocial personality, and conduct
disorder. For example, some biological accounts of antisocial and aggressive behavior
suggest they are an artifact of inherited or acquired deficits in frontal brain areas,
especially the orbitofrontal cortex, likely due to aggressive dyscontrol/disinhibition (e.g.,
Brower & Price, 2001), while others stress the impact of early child maltreatment
combined with an antisocial genetic predisposition (e.g., Caspi et al., 2002). The
“Attenuation Hypothesis” of Susman (2006) suggests that inherited vulnerabilities and
early life adversities predispose some children to an attenuation (reduction) of arousal as
a means of maintaining an equilibrium in their “internal metabolic milieu” in the face of
an unpredictable external environment. Such attenuation of endocrine physiology in the
stress system is considered to be a key mechanism in persistent antisocial behavior. Other
studies have linked environmental stressors that influence biology to aggressive and
violent behavior. For example, Liu, Raine, Venables, and Mednick (2004) found that
children who were malnourished at age 3 were relatively more aggressive and
hyperactive at age 8, more aggressive and prone to externalizing (acting out) behaviors at
age 11, and more hyperactive and more likely to exhibit symptoms of conduct disorder at
age 17.
A unified model of aggression: The General Aggression Model
The most recent comprehensive model of aggression, the GAM (Anderson &
Bushman, 2002), unifies a wide range of existing models, theories, and approaches. The
core assumption of the model – similar to the Cognitive Neo-Association Theory – is that
concepts, affects, memories, and action tendencies exist in an associative neural network,
and that human psychological processes and behavior, including aggressive behavior, are
primarily determined by what is activated within that neural network. For this reason, the
pattern of associations within the individual’s neural network is crucial in determining
behavioral tendencies, with highly associated groupings of attitudes, beliefs, knowledge,
expectations, affects, memories, and action tendencies – called knowledge structures –
playing a key role. Of course, every individual has a different neural network, and the
factors that influence its development are multitude.
Within this framework, the GAM explains the processes that lead up to and
account for an instance of aggressive behavior, including causal factors related to the
individual and the environment, and the internal psychological processes that occur
immediately prior to aggressive behavior. The GAM also allows for outcomes to feed
back into both the situation and the person’s own makeup.
The GAM is deceptively parsimonious (see Figure 1). Every instance of
aggression involves an individual person, with all their characteristics (e.g., biology,
genes, personality, attitudes, beliefs, behavioral scripts), responding to an environmental
trigger such as a provocation, an aversive event, or an aggression-related cue (lower
portion of Figure 1). These person and situation variables influence the person's present
internal state—cognitions, affects, and physiological arousal. Depending on the nature of
activated thoughts and feelings, and on how aroused the person is, the person’s
immediate response may be an impulse to aggress. The person may act on this impulse,
but if they have the time and cognitive resources to do so, and if the immediate response
is undesirable (e.g., it triggers an uncomfortable emotional response), a period of
appraisal and reappraisal will follow. Consequences are then thought-through, alternate
responses considered, and a more considered response made (see Figure 2 for an
expanded view of appraisal and decision making processes in the GAM). The resulting
behavioral action may or may not be aggressive. However, all actions feed back into the
immediate situation and also influence the person's psychological make-up (i.e., their
personality; see upper portion of Figure 1).
Biological
Modifiers
Environmental
Modifiers
Personality
Situation
Person
Proximate
Causes &
Processes
Present
Internal
State
Social
Encounter
Cognition
Affect
Appraisal &
Decision
Processes
Arousal
Thoughtful
Action
Impulsive
Action
Figure 1: General Aggression Model. From Anderson & Anderson, 2008. Reprinted by
permission.
Underpinning the apparent simplicity of the GAM is a highly detailed account of
known processes and findings from the aggression literature (see Anderson & Bushman,
2002, for a review). These relate to an extensive range of within-person factors and
possible triggers for aggression (a number of which are described in section 4 of this
chapter), as well as known internal cognitive and affective processes, and the
mechanisms by which behavior is reinforced and learned (i.e., long-term changes to
personality). A key assumption underpinning the GAM is that knowledge structures such
as scripts and schemas (groupings of knowledge, feelings, memories, perceptions and
notions about typical behavior that are centered around a particular theme) are person
factors that can not only impel a person to be aggressive in the moment, but can also
change to reflect our experiences (upper portion of Figure 1). Thus, experience leads to
changes in the type, content, and accessibility of knowledge structures.
Figure 2: Expanded view of appraisal and decision making processes in the GAM. From
Anderson and Bushman (2002).
Because of the large range of factors that can feed into the person and the
situation, and because the GAM gives no precedence to any internal processes (that is,
cognitions, affects and physiological arousal can individually or in any combination drive
an aggressive response), the GAM has the capacity to explain short- and long-term
aggression across a wide range of forms and functions, including along the three function
dimensions already noted (affect, goals, automaticity).
The efficacy of the GAM in explaining a wide range of aggressive phenomena is
demonstrated in a large and growing body of research, ranging from male-on-female
aggression (e.g., Anderson & Anderson, 2008) to the relation between personality
disorders and violence (e.g., Gilbert et al., 2015), to media violence effects (Anderson et
al., 2003), and even to rapid climate change on intergroup violence and war (Plante,
Allen, & Anderson, in press). Research using the GAM as a theoretical underpinning is
also breaking new ground when examining well-established findings. For example,
Bartholow, Anderson, Carnagey, and Benjamin (2005) demonstrated that the weapons
effect—the well replicated finding that the presence or photo of a weapon (e.g., gun) can
increase aggressive behavior— depends on a complex mix of life experiences and the
specific type of weapon displayed. Whereas hunters tended to associate hunting guns
with more positive thoughts about family gatherings (e.g., hunting with Dad), but
associated assault guns and hand guns with violence, non-hunters showed a significantly
different pattern of associations. Aggressive behaviors in the presence of different types
of weapons correspondingly differed for hunters and nonhunters. Other examples of how
the GAM has led to new research findings can be found in the citations above and in
subsequent sections.
3. Aggression research methodologies
Researching aggressive behavior is neither easy nor straightforward. Most notably
there are ethical constraints on studying aggressive behavior and there are appropriate
external validity considerations. Each aggression research methodology has its own set of
weaknesses and strengths that impact the interpretation of findings. However, it is
important to note that the weaknesses of each methodology can be overcome with the
strengths of another, allowing aggression researchers greater confidence where findings
converge across a range of methodologies (Warburton, 2013). This is characteristic of all
complex science domains, not something limited to aggression or psychology.
Experimental Aggression Studies
Experiments provide the strongest evidence that a particular factor may play a
causal role in aggression because they hold constant (as much as possible) the
experiences of participants, except their experience of an experimental manipulation
(such as watching a violent versus a non-violent video clip). Then, if the experimental
groups differ on average scores for a particular outcome (such as aggressive behavior),
one can be fairly confident that the manipulation (and not another factor) was responsible
for this difference. Experiments can be conducted in naturalistic settings, but are more
commonly done in laboratories.
The types of outcomes typically measured in aggression experiments are levels of
aggressive behavior, or the degree to which known aggression precursors (e.g.,
aggressive cognitions or feelings) are activated in participants. It is important to note that
aggression experiments are conducted within strict ethical guidelines and can only
measure fairly mild and short-lived forms of aggression (typically lasting 10-15 minutes
or less, in line with neural activation).
Measuring aggressive behavior has a controversial history, mostly due to
concerns that the measures are not valid or that the findings do not generalize. However,
several modern paradigms have been developed that are valid, reliable, and ethical. These
typically involve a contrived laboratory situation that allows participants to behave in a
way they believe will harm another, but in which no person is actually hurt. The
competitive reaction time paradigm measures the intensity and/or duration of aversive
“noise blasts” (originally, electric shocks) ostensibly delivered to an opponent. The
teacher-learner paradigm involves ostensibly delivering noxious stimuli (shocks, loud
noise) to another person. The hot sauce paradigm measures the amount of hot chili sauce
that participants believe they are making another person eat even though they know this
other person dislikes hot foods. The most recently validated laboratory aggression
measure is the tangram task help-hurt paradigm (e.g., Saleem, Anderson, & Barlett, 2015),
which measures the extent to which the study participant attempts to help or hinder
another person's attempt to win a monetary prize. If these (and other) such methodologies
are undertaken with careful attention to experimenter scripts, cover stories, and possible
participant suspicions, they measure a genuine intention to harm another and are not
subject to biases such as the desire to please (or to confound) the experimenter. Research
also suggests that such measures can predict aggression in the outside world (e.g.,
Anderson & Bushman, 1997).
Personality and individual difference variables are often used as moderators or covariates in experimental investigations into aggression. For example, the experimenter
may be interested in whether there is an interaction whereby participants high in a
particular trait (such as narcissism) are disproportionately influenced by the experimental
manipulation. The experimenter may also wish to see how much a manipulation increases
aggression over and above factors such as aggressive personality. In this case, an
appropriate measure (such as trait aggression) may be entered as a covariate and thus held
constant in analyses.
Non-experimental aggression studies
Non-experimental research, such as observational, cross-sectional, and
longitudinal studies have the distinct advantage that they report on a wide range of realworld phenomena. The latter two methodologies can also be used to research longer-term
effects such as the factors that contribute to the development of an aggressive personality.
Because such studies do not involve researchers in creating real harm, but merely
measuring it, even extreme types of aggression can be assessed. Cross-sectional studies
(which measure a range of factors at the same point in time) have a number of issues that
restrict the interpretation of findings. First, it can be quite risky to draw strong causal
conclusions from cross-sectional data because of possible confounds. Nevertheless, such
studies can provide valuable real world data that test causal theories and plausible
alternative explanations about aggressive behavior and which complement experimental
findings. Cross-sectional studies also provide valuable findings about the personality and
individual difference correlates of aggression. A second issue common in cross-sectional
studies (and that also exists in many experimental and longitudinal studies) concerns the
use of self-report data. Such data can be of poor quality if the respondent has poor selfknowledge, does not understand the questions, is in an unhelpful frame of mind, or
responds in a biased way.
In longitudinal studies key variables are measured at multiple points in time in the
same people. This is particularly important because the development of aggression can be
mapped in individuals across time, and correlated with a range of within-person and
environmental variables. It is also possible to (cautiously) infer some degree of causality
to some predictors due to (a) recent advances in statistical techniques, and (b) the logical
impossibility of a later measured factor causing differences in an earlier one. For example,
aggression measured at age 12 cannot have caused a person to be malnourished at age 2.
Observational measures of aggression have a number of advantages over selfreport measures, and are commonly used in all three major types of studies (experimental,
cross-sectional, longitudinal). Observational measures use data from observing the
behavior of a target, often in their natural environment, thus having good ecological
validity. Meaningful observations can also be made of populations that can be hard to
research using other methods (e.g., young children, those with poor language skills or
limited cognitive ability) or participants who might be reluctant to show true behaviors in
other research settings. This method has some drawbacks (e.g., low incidences of many
aggressive behaviors in an observed environment, subjective biases in rating behaviors),
but these can often be overcome by creating clear and comprehensive study guidelines,
providing detailed information about the behaviors to be coded (e.g., hitting, pushing,
shoving, and slapping might all be coded as physical aggression), thoroughly training
raters, and using multiple raters. Gaining corroborative data from multiple sources (e.g.,
peers, parents, teachers) can also strengthen observational findings.
Aggression and brain imaging
The growing availability of brain scanning techniques has fueled a surge in brain
scanning research involving aggression. This approach has multiple advantages, most
notably its applicability to people of all ages and abilities, the low likelihood that
participant responses will be intentionally biased, and its utility to measure some
phenomena that are hard to assess by other methods (such as desensitization).
Disadvantages include cost, small sample sizes, and for functional magnetic resonance
imaging (fMRI) studies, issues with image-averaging software producing false positives
(see Eklund et al., 2016). Studies utilizing fMRI identify brain activity by measuring
changes to blood flow and can accurately pinpoint the location of brain activity.
Techniques that measure brainwave activity (electroencephalograpgy [EEG] and
magnetoencephaloghraphy [MEG]) can provide readings accurate in time to small
fractions of a second.
In aggression research, participants are typically scanned while experiencing
aggression-eliciting stimuli or a control condition. Data may reveal changes that occur
over time (e.g., desensitization to violence), differences in brain activation between
groups (e.g., between those playing a violent or non-violent video game), and differences
in responding for people with different characteristics as measured by pre-test
questionnaires. One area of aggression research with a significant and growing number of
brain-imaging studies is the study of violent media. For example, Gentile, Swing,
Anderson, Rinker, and Thomas (2016) found that when habitual non-violent gamers
played a violent video game there was increased activity in the brain’s emotion centers,
whereas habitual violent gamers actively suppressed the same regions when playing the
violent game. Mathews et al. (2005) found differences in frontal lobe activation for youth
diagnosed with disruptive behavior disorder (i.e., aggressive youth) relative to
nonaggressive controls, and also found corresponding deficits in a counting Stroop task
that assesses executive control. Interestingly, normal control participants who had a
history of high media violence exposure had executive control deficits similar to the
disruptive behavior disordered participants. Bartholow, Bushman, and Sestir (2006)
found that high video game violence exposure predicted desensitization to violent photos
(as assessed by amplitude of the P300 EEG component), which in turn predicted later
aggressive behavior. Other EEG studies have likewise found that individual differences
in media violence exposure predicted proactive executive control deficits (assessed with
ERPs and Stroop tasks) and suppression of activity in emotion-related brain regions (e.g.,
Bailey, West, & Anderson, 2011).
4. Research findings: Factors that cause aggression
It is important to note that the factors and research described in this section all
feed into the GAM as either within-person factors that impact one's predisposition to
aggress, or as situation factors than can trigger aggression in many or most people.
Development of aggression and stability over time
When people are compared one to another, individual aggressiveness appears
fairly stable such that children who are more aggressive than their childhood peers are
also likely to be more aggressive than their adolescent and adult peers later in life
(Bushman & Huesmann, 2010). However, within the person, aggressive behavior is
usually not static across the lifespan. In most people, the frequency of physical
aggression typically follows a curvilinear path across development, peaking in the toddler
years, and then decreasing across childhood, adolescence and adulthood. However,
individual differences in these trajectories have been mapped in several large longitudinal
studies, which consistently find a small subset of people whose aggression continues at
high levels through development and into adulthood, and most of whom are male (e.g.,
Côté et al., 2006; see also Fontaine, this volume, on the development of antisocial
behaviors and Vachon and colleagues, this volume, on externalizing behaviors). A
number of factors predict whether individuals will follow this high aggression
developmental path, most notably hostile and inconsistent parenting, maternal rejection,
poor family functioning, fearlessness in the child, low income, and social disadvantage.
Sensitive and involved parenting seems to be a key protective factor.
Personality factors
Trait aggressiveness. Trait aggressiveness has typically been conceptualized as
comprising higher than average everyday levels of physical aggression, verbal aggression,
hostility, and anger (e.g., the widely used Aggression Questionnaire [AQ]; Buss & Perry,
1992). This approach departs somewhat from the now widely accepted definition of
aggression as a behavior, and an argument could be made that only aggressive behaviors
should be assessed. Nevertheless, most studies of trait aggression, many of which use the
AQ, find that those high in this multifaceted trait behave more aggressively, even when
minimally provoked.
Trait Anger. Trait anger is typically conceptualized in terms of an individual’s
tendency to feel anger more intensely, more often and for a greater duration than their
peers. Compared to others, individuals high in trait anger are more sensitive to
provocations (e.g., feeling they have been treated unjustly or have been unfairly
criticized) and are more likely to respond with aggression. Although trait anger usually
correlates positively and robustly with trait aggression, and shares some conceptual
overlap with widely used formulations for trait aggression (e.g., the AQ), trait anger is
considered a distinct and separate construct.
Trait Irritability. Trait irritability shares some conceptual common ground with
trait anger and trait aggression, but is treated as a separate trait by some researchers (e.g.,
Caprara et al., 1986). Although individuals high in this trait are angrier in general, there is
also a noteworthy sensitivity, with Caprara and colleagues describing a “tendency to react
impulsively, controversially, and offensively to the least provocation” (p. 84). Trait
irritability is positively associated with aggressive behavior and trait aggression.
Personality traits – the “Big Five” and the “Big Six”. Aggressive behavior has
been linked to some traits from both the “Big Five” model of personality (Agreeableness,
Conscientiousness, Extraversion, Neuroticism, and Openness to experience; see McCrae,
this volume) and the Big Six (HEXACO model) of Lee and Ashton (2004) - Extraversion,
Agreeableness, Conscientiousness, Emotionality (similar to Neuroticism), Openness to
experience, and Honesty/Humility. In terms of the Big Five, a recent meta-analysis
suggests that aggressive behavior has the strongest (negative) correlation with
Agreeableness (i.e., it aligns with its opposite pole, Antagonism), most notably with the
facets of straightforwardness, compliance, and altruism. Aggression also has a fairly
robust negative correlation with Conscientiousness (particularly the facet of deliberation),
a positive correlation with Neuroticism (especially the angry hostility facet), and a small
negative correlation with Extraversion (mainly the warmth facet; see Jones, Miller, &
Lynam, 2012). Perhaps the most interesting higher-order personality trait emerging as an
aggression predictor is Honesty/Humility from the HEXACO, which explicitly contrasts
pro-social and antisocial behavior and seems to be a consistently strong predictor of
aggression, bullying, and aggressive personality styles such as narcissism and
psychopathy.
Emotional susceptibility. This trait is conceptualized as a stable tendency to feel
distressed, inadequate, and vulnerable to perceived threats, and to experience negative
affect and upset when insulted or subjected to personal attacks. Emotionally susceptible
people tend to be more aggressive than their less susceptible peers, especially when
provoked.
Callous and unemotional personality traits. The three personality styles under
this umbrella – psychopathy, Machiavellianism, and narcissism – have been dubbed the
“Dark Triad” (Paulhus & Williams, 2002). All three are linked with lack of guilt, absence
of empathy, curtailed emotional responding, and callous disregard for others. However,
despite this conceptual overlap, the three styles also have distinct elements: narcissists
need to be admired and are particularly susceptible to ego-threats; psychopaths are
impulsive, pleasure seeking, and unconcerned with consequences; Machiavellians can
delay gratification and tend to behave in the service of their goals rather than their
appetites. Reviews of the research suggest that callous and unemotional traits are fairly
stable across both childhood and adolescence, and predict high levels of aggressive and
antisocial behavior across the lifespan (see Frick & White, 2008).
Although all three callous-unemotional personality styles are robustly and
positively correlated with aggressive behavior, this aggression typically differs in
function (and sometimes type) across the three groups. Narcissists tend to respond
aggressively to threats that might undermine their grandiose public image (e.g., to insults,
public humiliation, or other threats to their inflated ego; Thomaes, Brummelmann, &
Sedikides, this volume). A lesser known group of narcissists, covert narcissists, have a
similar sense of underlying entitlement and superiority to overt narcissists, but are more
emotionally vulnerable and rejection sensitive. They are less grandiose in public but no
less aggressive in the face of perceived ego (and other) threats, with their aggression
likely fueled by high levels of trait anger, trait hostility, and shame-proneness (Warburton
et al., 2008). Psychopaths are aggressive across a wide range of situations and use
aggression for many reasons. They respond aggressively to provocations and perceived
threats, will use aggression in a purely instrumental way to obtain something they want,
and will sometimes harm others just for personal pleasure. Machiavellians tend to use
aggression strategically and instrumentally, and have little compunction about harming
others either personally or through others. However, the Machiavellian – ever alert for
negative consequences – will often use others to carry out harm to their intended victims
so that they cannot be held personally responsible.
The shame-prone individual. The shame-prone individual can be conceptualized
as having deep-seated feelings of whole-of-self inadequacy and a fundamental fear that
their flaws will be exposed to others (contrast this with guilt, which typically involves
remorse about a specific act). Although shame can drive people to pro-social and
appeasing behaviors, it is also clear that shame can elicit both anger and aggression.
Theorists dating from the 1970s have described an escalating upward spiral whereby
shame causes anger and this anger then feeds shame and so on. Researchers have also
consistently found that state shame can elicit aggressive behavior, and that shameproneness is positively correlated with levels of verbal and physical aggression (e.g.,
Tangney et al., 1996).
Impulsivity, executive control, and self-control. Impulsivity is a key
temperament variable on which humans differ and individual differences are noticeable
beginning in infancy. Impulsivity is a reliable predictor of human aggression, presumably
because those who are highly impulsive tend to act on aggressive impulses without an
effective internal mechanism to curb those impulses or think them through. Indeed,
higher levels of executive control, emotional control, and self-control are all linked with
lower levels of aggression (see Moffitt et al., 2011; see also Hoyle & Davisson, this
volume, on self-regulation). In contrast, individuals who have less control over their
emotions and their behavior tend to be more aggressive (and sometimes more violent).
For example, research shows robust links between impulsivity and domestic violence,
and a number of taxonomies of domestic violence perpetrators include a group
characterized by poor impulse control (e.g., Edwards et al., 2003).
Dissipation/Rumination. Although most people have some capacity to dissipate
their feelings of anger and hostility after being upset or provoked, at the other end of the
spectrum are a smaller group of people who tend to ruminate on upsetting events,
maintaining and sometimes increasing these negative feelings. High ruminators (low
dissipators) tend to be more aggressive by disposition, and are particularly likely to be
aggressive during a period of hostile rumination if there is some sort of trigger for
aggression, such as a minor annoyance (e.g., Bushman et al., 2005).
Intelligence. Research on IQ and aggression is somewhat sparse but a number of
studies have found links between low IQ and higher levels of aggression in children (e.g.,
Huesmann, Eron & Yarmel, 1987). More recently, links between lower verbal
intelligence and aggression were found to be most pronounced in boys who also had poor
self-regulatory skills (Ayduk et al., 2007).
Personality disorders and aggression. Higher levels of aggression are common
in individuals with some personality disorders included in the Diagnostic and Statistical
Manual of Mental Disorders (DSM-5; American Psychiatric Association, 2013). High
levels of aggression are very common (and somewhat diagnostic) in individuals with high
levels of psychopathy and those with a clinical diagnosis of Antisocial Personality
Disorder (the DSM-5 diagnosis that is most similar to psychopathy). Also often
aggressive, though to a lesser extent, are those with a diagnosis of Narcissistic
Personality Disorder, Borderline Personality Disorder, and Paranoid Personality Disorder.
Aggression is sometimes linked with other personality disorders such as Schizotypal
Personality Disorder but these results are less clear (e.g., Gilbert et al., 2015).
Other “person” factors
A number of other factors within a person’s make-up have been shown to increase
the likelihood of aggressive behavior.
Gender differences in aggression. Research shows that males are typically more
physically aggressive and violent than females, with gender differences appearing early
in life and continuing across the lifespan. This is particularly evident in violent crime
rates across countries and time. It should be noted though, that women are as physically
aggressive as men when strongly provoked (Bettencourt & Miller, 1996). One somewhat
surprising finding is that in cross-gender encounters, girls are often more physically
aggressive than boys (Archer & Cote, 2005). Similarly, among heterosexual couples
women are slightly more likely to initiate physical aggression than men, even though
women are much more likely to suffer major injuries. This paradox has several
interrelated resolutions. It appears that women tend to use physical aggression as a signal
as to how upset or angry they are, and tend use less harmful tactics such as slapping,
kicking, biting, and throwing things. Men, on the other hand, tend to use physical
aggression to control and with the intention of harming, and tend to use more harmful
tactics such as beating up, choking, or shoving (e.g., Archer, 2000). In combination with
the fact that on average men have more body mass, upper body strength, and practice in
punching and fighting, these differences in goals and tactics account for the paradox.
Boys tend to be more verbally aggressive than girls, though this difference is not
as large as for physical aggression. This difference appears to persist well into adulthood.
In terms of relational aggression, studies often find few gender differences in adult
populations, but some child studies find girls are more relationally aggressive than boys
(Crick, Ostrov, & Kawabata, 2007).
Genetic predispositions and epigenetics. In line with the basic principles of
epigenetics, it is clear that while trait aggression has some genetic component, the
environment in which the genes are expressed, and other learned experiences, are also
developmentally important. Inherited characteristics are thought to account for perhaps a
quarter to a third of an aggressive predisposition (Tuvblad, Raine, Zheng, & Baker, 2009)
and over a dozen genetic markers have been linked with aggressive and antisocial
behavior. The links from genetic markers to aggression are rarely (if ever) direct, with
most relating to temperament variables (such as impulsivity), that are themselves linked
with higher levels of trait aggression. There are two fairly well researched genetic
markers of antisocial personality/aggression. A polymorphism in the 5HTTLPR
(serotonin promotor) gene is linked with attention deficits which are themselves linked to
aggression. A polymorphism in the monoamine oxidase A (MAOA) promoter gene
seems to express as antisocial behavior in children who also experience childhood
maltreatment. For example, in the sample tested by Caspi and colleagues (2002), 12%
had both the genetic risk and childhood maltreatment, yet accounted for 44% of the total
adult convictions for violent crime. Furthermore, 85% of those who had both risk factors
developed some form of antisocial behavior. In the absence of maltreatment, the
genotypic risk factor did not manifest itself behaviorally.
Hormones and neurotransmitters. The hormones most consistently linked with
aggression are testosterone, cortisol, and oxytocin. Because males are more physically
aggressive than women and also have testosterone levels typically about ten times those
found in females, and because both testosterone levels and aggression decline as men age,
there has been particular interest in testosterone, which has been linked with dominance
behaviors and aggression. Research has also shown that prenatal levels of testosterone
may influence brain organization and development, thus influencing a wide range of
brain functions that play a role in later aggressive tendencies. More recently it has been
suggested that high testosterone is most strongly linked with aggression if the individual
also has low levels of cortisol (the stress hormone) and the neurotransmitter serotonin,
which is linked with capacity for impulse control (e.g., Montoya, Terburg, Bos, & Van
Honk, 2012). Low basal levels of cortisol have also been independently linked to
antisocial behavior in children, adolescents, and adults (see Susman, 2006), and
concentrations of oxytocin in cerebrospinal fluid have been shown to have an inverse
relationship to a life history of aggressive behavior (Lee et al., 2009). Findings linking
estrogen and progesterone to aggression have been mixed, with lower levels of both
hormones associated with higher aggression in some studies (see Buchanan, Eccles &
Becker, 1992) and higher levels of estrogen linked with greater aggression in others (e.g.,
Finkelstein et al., 1997). Paradoxical findings with oxytocin have also been reported (see
Crespi, 2016, for a review).
The neurotransmitter most consistently linked to aggressive behavior is 5hydroxytryptamine (5-HT), serotonin, which is linked with the inhibition of impulses.
Low levels of serotonin are thought to facilitate aggression via a reduced capacity to curb
aggressive tendencies. The role of the catecholaminergic neurotransmitters dopamine and
norepinephrine are less clear. Animal studies and some evidence in humans suggest that
higher levels are linked with a lower threshold to respond to environmental stimuli with
aggression (e.g., Volavka, Biler & Nolan, 2004). However, studies have also shown
comparatively low catecholaminergic neurotransmitter activity in aggressive, antisocial,
and psychopathic individuals (e.g., Susman, 2005). The neurotransmitter Gamma
Aminobutyric Acid (GABA) plays an important role in rodent aggression (with more
aggressive rodents often having higher GABA levels) but this neurotransmitter is as yet
understudied in human aggression.
Neuroanatomy and neural functioning. As already noted, persistent antisocial
behavior and aggression have been linked to dysfunction in parts of the brain that
facilitate executive functions such as thinking through consequences and inhibiting
impulses (e.g., frontal lobe injury or deficits). Structural and functional deficits in three
specific regions are consistently implicated as being instrumental in antisocial behavior
and aggression - the Orbitofrontal Cortex and Anterior Cingulate Cortex, particularly in
the right hemisphere, and the Dorsolateral Prefrontal Cortex in the left hemisphere (see
Yang & Raine, 2009, for a review).
In addition, there are robust findings showing that people high in trait aggression
and antisocial behavior tend to have low arousal in both the central nervous system
(CNS) and autonomic nervous system (ANS), and seem to have an attenuated stress
response system (e.g., Susman, 2006). For example, Raine and colleagues (1990; 1997)
showed that low tonic heart rate at age 3 predicted aggression at age 11, and that lower
cortical (EEG) arousal, lower cardiovascular arousal, and lower electrodermal arousal at
age 15 all predicted criminality in male adults. In addition, trait aggression has been
linked to lower levels of stress hormones (such as cortisol) as well as with low levels of
other byproducts of the stress response system.
Environmental cues and triggers for aggression
Provocation. Arguably the single most important trigger for aggression (and in
particular reactive aggression) is provocation by another person (Bettencourt et al., 2006).
It is important to note that the aggressor does not need to be provoked directly. It is often
sufficient for the person to be provoked by social rejection, unkind rumors, having
provocative comments posted online, or a range of other provocative actions not
delivered face to face. Indeed, the provocations do not even need to be real. Simply
believing that someone has harmed you or thwarted an important goal is sufficient to
inspire aggression.
Weapons. Most individuals conceptually link weapons with aggressive behavior
in their neural network, likely because they almost exclusively see weapons used to hurt
others, whether in real life or on screens. As discussed earlier, it is not surprising that a
“weapons effect” is consistently found, whereby people who view a real or virtual
weapon tend to have aggression-related cognitions primed in semantic memory, and are
more likely to behave aggressively. And as noted earlier, the effect depends on what
types of cognitions are primed by the weapon, which in turn depends on the individual's
life history (Bartholow et al., 2005).
Violent environment. Although most individuals are raised in relatively peaceful
environments, a substantial number of people grow up in homes, neighborhoods, or
countries where they are exposed to considerable violence. Such individuals will have an
associative neural network that includes many aggression-related concepts and
knowledge structures, including aggressive scripts for how to manage conflict. Thus it is
not surprising that people from violent environments have a higher likelihood of
becoming aggressive individuals (e.g., Aguilar, Sroufe, Egeland, & Carlson, 2000).
Violent media. Violent environments do not need to be physical to influence
behavior – virtual environments also impact the way people think, feel, and behave.
There has been considerable research on the impact of violent media such as television,
movies, music, and video games, literally hundreds of studies across hundreds of
thousands of participants. Findings converge across all major research methodologies to
find that violent media exposure increases the likelihood of aggressive behavior and
causes desensitization to violence in both the short- and long-term (Warburton, 2014). In
addition, greater exposure to media violence has been linked to hostile biases in thinking,
increases in aggressive thoughts and feelings, and decreases in empathy and pro-social
behavior (see Anderson et al., 2003; 2015; Krahe et al., 2012 for reviews).
Environmental stressors. Factors in (or caused by) the environment which
humans find aversive or stressful tend to increase the likelihood of aggression. The list is
long, but the better researched factors include experiences that cause physical pain, hot
temperatures, offensive or unpleasant odors, and loud or aversive noises. Such factors are
typically more likely to elicit aggression where the individual has little or no control over
them (e.g., Donnerstein & Wilson, 1976).
Anonymity. Humans are typically more likely to aggress if they can do so
without negative consequences, and anonymous aggression is one way a person can harm
another without obvious costs. Thus, it is not surprising that anonymity, in some
circumstances, increases the likelihood of aggression. Hurting another anonymously also
allows the aggressor to hurt the target without personally engaging with them, thus
increasing the likelihood of “de-humanization” and a consequent lessening of the
restraints on antisocial behavior normally accorded to other humans (see Haslam &
Loughnan, 2014, for a review).
Social rejection. Social inclusion and belonging are fundamental human needs.
Thus, when people feel excluded, rejected, unsupported, and/or lonely they are typically
motivated to seek re-inclusion and acceptance, often through pro-social and appeasing
behaviors. However, rejected individuals also tend to feel hurt, angry, and hostile toward
their rejectors, and a concurrent urge to lash out at those who have been rejecting or to
simply hurt someone else is also common. The social exclusion-aggression link is now
well demonstrated, particularly in circumstances where the person can do so without
harming their social position or where they perceive little control over their situation (e.g.,
Warburton & Williams, 2005; Warburton, Williams, & Cairns, 2006).
Interestingly, there is growing evidence that feeling isolated, unnoticed, powerless,
and insignificant can yield both anger towards those who appear responsible for this state
of affairs and a search for significance and meaning that can be perverted into violent
action. This appears to be the case in many instances of mass shootings and bombings,
including suicide bombings. Indeed, both domestic and international terrorism appear to
be related to such a "quest for significance" (Kruglanski & Orehek, 2011). People trying
to recruit members to their extremist causes (again, foreign or domestic examples exist)
target individuals and groups who have experienced major loss of meaning, personal
power, and/or connectedness, and provide them with the missing meaning in social life
and (often) death.
Substances. The link between alcohol intoxication and aggressive
behavior is robust in men and in women, both inside and outside of the laboratory. In
addition, alcohol intoxication is implicated in a substantial proportion of violent crimes,
including murders, assaults, rapes, and incidents of intimate partner violence. Alcohol
itself does not cause the person to be violent, but it has a complex impact on the human
brain and these processes tend to disinhibit aggressive impulses. Thus, people who are
predisposed to behave aggressively are most likely to be aggressive when intoxicated
(Giancola, 2000). Also, these effects are most likely to occur among individuals who
believe that alcohol causes aggression and that it provides them with an excuse that
society will accept. Aggression has also been linked with other substances that cause
disinhibition and/or an increase in physiological arousal, such as stimulants,
amphetamines, and methamphetamines.
5. Internal processes during aggression
Having examined within-person factors that increase the predisposition to aggress
and possible environmental triggers and cues for aggression, it is now important to
examine the processes that mediate aggression, those factors in the GAM related to the
individual’s “present internal state” immediately prior to aggression.
Emotion/affect. As already noted, a number of historical accounts of aggression
(e.g., the Cognitive Neo-association Theory) place considerable emphasis on the role of
negative emotions in causing aggressive behavior, and a number of such emotions have
been identified. The role of frustration was detailed earlier when describing the theory of
Dollard et al. (1939) and frustration is a well-documented precursor to aggression. The
role of anger in aggression is also well established, although the pathway from anger to
aggression is not always straightforward. Key ways that anger facilitates aggression
include reducing inhibitions, narrowing attentional focus to cues for aggression, and
alerting people to cues for potential threats (e.g., Anderson & Bushman, 2002). Shame
has also been linked to increases in aggression, as have feelings of humiliation. Indeed,
there is evidence that some acts of mass violence are partly motivated by feelings of
humiliation. Links between jealousy and aggression are also robust, particularly in
intimate relationships. Enacting aggressive behavior is also related to subsequent
reductions of some positive emotions that are protective for aggression, most notably
sympathy and empathy. It has also been shown that anticipated emotions (not just
currently experienced emotions) can determine whether an individual will be aggressive.
Cognition. In recent decades, aggression researchers have been increasingly
interested in the cognitions that are activated during an instance of aggression. Studies
have shown that holding and activating a range of hostile attitudes, beliefs, expectations,
perceptions, concepts, schemas, and scripts can increase the likelihood of aggressive
behavior. This activation may be below the threshold of awareness or in conscious
awareness. Either way, the activation of aggressive cognitions can elicit behavioral
aggression through the priming of aggression-related action-tendencies, the activation of
aggressive scripts for behavior, or through responding aggressively to cues from the
environment that have been interpreted through the lens of hostile biases.
Arousal. Arousal, be it physiological or emotional, does not cause aggression
directly but can facilitate aggression in several ways. The most common method by
which arousal facilitates aggression is by impelling a person to act on an aggressive
impulse without taking the time and/or using the cognitive resources needed to think
through the consequences of that action. Arousal may similarly impel a person to
aggressive action, with little conscious thought, as part of a fight or flight response. It is
also possible that over-arousal is experienced as unpleasant and may elicit aggression in
much the same way that other aversive experiences do. Finally, in line with cognitive
labelling findings and excitation transfer theory, aggression would be more likely in
individuals who are aroused and who attribute that arousal to anger. Also note that
abnormally low levels of arousal could, in theory, facilitate aggression due to the
person’s low energy and motivation sapping inhibitory control over aggressive impulses.
6. A “risk and resilience” approach to aggression and violence
As the previous sections show, there are many factors within the individual that
can influence their predisposition to aggress. Similarly, there are numerous factors from
one’s environment that can trigger aggression, and many internal processes that can cause
the person to respond to an environmental cue with aggression. One way of thinking
about how these predisposing factors can impel an individual to actual aggression is to
take a “risk factor” approach. This approach is widespread among aggression scholars,
and emphasizes one key premise: for all but mild forms of aggression, no single causal
factor is ever necessary or sufficient. From this perspective, every instance of moderate
aggression or violence occurs because of a confluence of multiple “risk factors” for
aggression, in tandem with insufficient “protective factors” to inhibit aggression.
Consequently, when an individual has a greater number of risk factors for aggression, or
when some of the risk factors have a very strong effect, then that person has a greater
likelihood of aggression, especially if protective factors for aggression are few (e.g.,
Gentile & Bushman, 2012). While this approach makes intuitive sense and helps make
sense of serious societal issues such as domestic violence and mass killings, it also
emphasizes the large number of factors that need to be taken into account, and the
amount of work needed to understand how the multiple causes of aggression factors
interact.
7. Treatment of aggression
Treatments for aggressive behavior typically arise within specific systems where
aggressive behavior is common and problematic—for example, bullying in schools,
rehabilitation of violent offenders, and managing aggressive or violent consumers of
mental health services. Treatments differ widely because there is so much variability in
the contexts involved, in the types of aggression or violence being treated, and in the
treatment populations. Although some pharmacological treatments are used (e.g., anticonvulsants, mood stabilizers), only psychological treatments are examined here. These
look to find the psychological causes of aggression and then find ways of ameliorating
their impact. Regardless of the treatment used, the risk and resilience approach is a
helpful adjunct. Therapists often seek to identify both risk and protective factors for
aggression in the person’s makeup and environment, and seek ways, with the client, to
reduce risk factors and increase protective factors.
Cognitive Behavioral Therapy (CBT)
Many individual and group therapies for aggression take a CBT perspective. In
this mode of treatment the therapist asks questions that search for commonalities in (a)
the circumstances that typically lead to aggression, (b) the actual instances of aggression,
and (c) the consequences of aggression. Are there particular triggers and cues for
aggression? Does the person tend to have hostile thoughts and biases? Is the aggression
rewarded in any way? What is the person feeling before, during and after aggression?
Therapy would then focus on reducing aggressive behavior by reducing exposure to
aggression triggers, changing unhelpful attitudes to aggression, challenging aggressive
cognitive biases, emphasizing the negative consequences of aggression, developing
conflict resolution strategies, finding ways to interrupt an escalation into aggressive
behavior, and finding alternative behaviors to aggression in the situations where
aggression has typically been most likely.
Anger management
Anger management courses are often a key component of rehabilitation programs
for aggressive or violent individuals. These help the individual to identify the situations
that precede anger and aggression, and factors that escalate their anger. Management
typically involves changing the contingencies between typical triggers and subsequent
responses, slowing down the escalation of anger, stress reduction, and finding effective
strategies for personal calmness, emotional control, and dissipating angry feelings in a
constructive way.
Family systems approaches
Family systems approaches are built around the core assumption that families and
their patterns of communication and interaction profoundly affect the individuals within
that family. When dealing with aggression, therapists from this approach emphasize the
interactions between people in the aggressive person’s network, and the ways that these
interactions might contribute to (or moderate) aggressive behavior. From this perspective,
aggression in a child might be managed through working with the child’s family to
develop interactions that are more stable and nurturing, and that establish clear and
respectful boundaries around acceptable behavior. Multi-systemic therapies meld family
systems-based interventions with CBT, and are typically used as part of intensive
interventions for serious cases of chronic aggression and violence.
Other approaches
Motivational approaches (e.g., “motivational interviewing”) emphasize assisting
the aggressive or violent client to understand the consequences of their aggression, and to
come to a point of being internally motivated to change their behavior. Individual
treatment may also focus on the root causes of aggression rather than the behavior itself.
For example, psychodynamic approaches, schema therapy, and other forms of
psychotherapy may focus on treating the sorts of traumatic experiences, acquired
maladaptive schemas, or forms of psychopathology that underlie aggression.
Multimodal approaches
For children, adolescents, and adults with serious aggression and violence issues,
individualized, multi-modal approaches are often most effective. For offender
rehabilitation programs this may take the form of multiple treatment modules that tackle
a wide range of factors that contribute to the violent behavior of participants. For highly
antisocial youth and young adults it may take the form of intensive treatment packages
that involve working with the youth’s family, school, and community, individual
psychological treatment using a range of therapeutic modalities, and assistance in key life
areas such as employment, training, and financial management.
8. Evidence based practice and policy
Research on aggression and violence has progressed tremendously in the last 30
years. We know what the major risk factors are, and have discovered that many of them
are malleable. That is, the pessimism that dominated the views of many scholars and
policy makers in the aftermath of two world wars and that accompanied the dramatic rise
in violent crime rates is not only self-defeating, but it has been shown by careful research
to be invalid. It is not the case that aggressive or violent individuals are born that way
(although genetics plays a role), or that there is nothing that society can do reduce the
frequency of unwarranted and unwanted aggression and violence. Even the long-held
belief that by the time a person reaches adulthood, violent tendencies are fixed and
therefore un-fixable has proven false (Bailey, Sehgal, Coscia, & Shelton, 2015; Dvoskin,
Skeem, Novaco, & Douglas, 2012; Henggeler & Sheidow, 2012).
Aggression (and violence) researchers from many social and behavioral sciences
have identified numerous risk and protective factors that society can influence positively.
Some of these are as direct as providing good prenatal nutrition to pregnant women, and
postnatal nutrition to infants and children. Others are more complicated for political
reasons, but also are backed up by research (e.g., providing children and youth with a
better media diet, one that discourages aggression rather than encourages it, that promotes
nonaggressive problem solving, and that glorifies other types of accomplishments; see
Warburton, 2012). Similarly, there are ways to restructure schools, work environments,
and prisons so that they encourage positive, prosocial growth. We know how to attack the
problems of bullying. We know how to intervene with violent juvenile offenders before
they end up in prison. We even know how to treat habitual violent offenders so as to
reduce the likelihood of recidivism (e.g., Dvoskin et al, 2012).
A major obstacle, however, continues to be a significant disconnect between what
the aggression/violence scientific community knows and what public policy makers
(especially politicians) and the general public know and are willing to do. Instead of
serious, scientifically-based prevention, intervention, and treatment programs, modern
society has received simple yet persuasive solutions that have no basis in theory or
evidence, such as midnight basketball programs, “just say no” campaigns, and “scared
straight” and boot camp interventions. What is needed is a major change in attitude and in
understanding of what good science can do toward solving human problems. Modern
societies need to use what aggression science already tells us about changes that are
likely to improve human welfare, and to demand that all public policy changes designed
to produce such change be evaluated by competent scientists so that policies that do not
work can be dropped or changed, and those that do work can be strengthened and
improved.
9. Future directions for research
Although aggression science has improved dramatically over the past three
decades, additional research and more comprehensive theories are needed to improve the
precision of our predictions. This will require cross-disciplinary collaboration and the
creation of new areas of research. For example, social neuroscience is in its infancy, and
though important strides have been made, many more are yet come. In 10 short years, the
media violence domain has benefited from a call for more integration in this area (e.g.,
Carnagey, Anderson, & Bartholow, 2007), as the sample of brain imaging findings noted
earlier attests. Additional “basic-science” types of research on aggression will, in the end,
lead to better applications.
One of us (CAA) recently had the opportunity to meet with National Academy of
Science members to discuss media issues in children's health. One of the outcomes of
many meetings with many scientists involved with this project is a proposal for a
"large-scale (i.e., at least 50,000 participants) developmental study that
incorporates state-of-the-art measures of all known major child and adolescent
risk and resilience factors for the development of violent behavior tendencies. The
study should follow the same large sample of children from a very early age (e.g.,
2 years) though early adulthood (e.g., 30 years), and should include known
biological, environmental, and social pre- and post-natal factors that affect the
development of healthy, productive adults (versus unhealthy, unproductive,
antisocial adults). The study should over-sample from high-risk populations"
(Anderson et al., under review).
By assessing "all known" relevant factors, including those from genetics, neuroscience,
economics, nutrition, biology, psychology, criminology, and sociology, it should be
possible to estimate which subpopulations are at risk from which risk factors, and which
protective factors are most likely to be effective in promoting socially healthy children
and adults. Such a study would surpass previous longitudinal studies, which have fallen
short in terms of the quality of measures used to assess some of the key risk factors, most
notably (perhaps) measures of screen media usage.
A third direction for future research is better and larger intervention, prevention,
and implementation studies. There are numerous small scale studies that have
demonstrated positive effects, in schools, communities, and even with prison populations.
But we need more large scale studies that implement these programs, to test their
generalizability, efficacy, and cost-benefit ratios. Such studies should eventually make it
easier to get the political support to fund widespread change, as the few such analyses
that have been done tend to show that in the long run it costs less to produce healthy
(physical, social, emotional) citizens than it does to imprison or hospitalize unhealthy,
antisocial individuals.
We hope that this chapter helps a wide range of scholars to understand how
important individual differences in aggressive tendencies develop, and how individual
difference scholars can use this knowledge to create healthier social systems.
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