Genetic and environmental
factors associated with
intelligence test performance.
Video
Genetic and environmental factors associated
with intelligence test performance.
• including the influence of culture.
• So if it is genetic, hereditary then our intelligence is purely down to
our parents genes!
• Those who believe in the importance of heredity draw a distinction
between the genotype and the phenotype. The genotype is the
genetic inheritance whereas the phenotype consists of an individual’s
observable characteristics. So far as intelligence is concerned, we
can’t assess the genotype. All we can do is assess the phenotype by
administering an intelligence test.
Hereditary and the environment
• However, the reality is we cannot separate out the effects of
hereditary and environment because our genetic makeup influences
the types of environmental experiences we have.
Three types of interdependence
• Plomin (1990) identified three types of interdependence between
genetic factors and environment:
1.
Active covariation: occurs when children of differing genetic
ability look for situations reinforcing their genetic differences (e.g.
children of high genetic ability reading numerous books).
2.
Passive covariation: occurs when parents of high genetic ability
provide a more stimulating environment than parents of lower genetic
ability.
3.
Reactive environment: occurs when an individual’s genetically
influenced behaviour helps to determine how he/she is treated by
other people.
Twins and how they help
• Monzygote (one egg). Genetically the same so should have same
intelligence (genetic). If they have the same upbringing then hard to
prove environmental or genetic factors but adoption or being brought
up apart can show if one is more important than the other.
• Zygotic twins: (50%) Different eggs. Brought up the same may show
the environmental factors have greater influence or in fact that
genetic predisposition does.
Research evidence for genetic factors
• Bouchard and McGue (1981) reviewed 111 studies, and reported that the mean
correlation for identical twins was +.86 compared to +.60 for fraternal twins.
• McCartney, Harris, and Bernieri (1990) reported similar findings from a later
analysis of numerous studies: the mean correlation for identical twins was +.81
compared to +.59 for fraternal twins.
• Bouchard and McGue (1981) found that the mean correlation coefficient for
identical twins brought up apart was +.72. Identical twins brought up apart
should be very similar to each other in IQ if genetic factors are very important.
Thus, the +.72 seems to provide fairly convincing evidence for the importance of
both genetic and environmental factors. The finding that the correlation is higher
than that for fraternal twins brought up together suggests the importance of
genetic factors. The finding that the correlation (+.72) is lower than that for
identical twins (+.86) suggests the importance of environmental factors.
Research evidence for genetic factors
• Bouchard et al. (1990) found similar findings to the above study as they
studied more than 40 adult identical twin pairs separated at a mean age of
5 months, and found their IQs correlated +.75. The similarity of the
correlations supports the reliability and validity of the genetic basis of
intelligence.
• Mackintosh (1998) reviewed the evidence based on heritability measures.
He concluded that between 30% and 75% of individual differences in
intelligence in modern industrialised societies are due to genetic factors.
• Brace (1996) found that the heritability of intelligence was much higher
among people living in affluent white American suburbs than among
people living in American urban ghettoes. This is because the favourable
environment experienced by those in the suburbs meant individual
differences in intelligence depended mainly on genetic factors.
Research evidence for genetic factors
• Horn (1983) reported findings from the Texas Adoption Project, which
involved almost 500 adopted children. The correlation between the
adopted children and their biological mothers was +.28, and between the
adopted children and their adoptive mothers was even lower at +.15. Both
of these correlations are very low but they do suggest a greater role for
heredity as the correlation between biological relatives was higher than
between adopted relatives.
• Loehlin, Horn, and Willerman (1989) found there were some differences in
the findings when the adopted children were tested again 10 years later.
Now the children showed an increased correlation with their biological
mothers, but a reduced one with their adoptive mothers. Shared family
environment between the adopted children and their adoptive mothers
was reduced in importance. In contrast, genetic factors had a greater
influence on the adopted children’s intelligence than 10 years earlier.
Research evidence against genetic
factors and so for environmental factors
• Loehlin and Nichols (1976) point out that the differences between MZ and
DZ twins may not be solely due to genetic factors because identical twins
are treated in a more similar fashion than fraternal twins. This can include
parental treatment, playing together, spending time together, dressing in a
similar style, and being taught by the same teachers. Thus, the differences
in intelligence may be due to environmental, rather than genetic, factors.
• The prenatal environment may also explain the differences in intelligence
between MZ and DZ. Two-thirds of identical twins (MZ) share a placenta
whereas fraternal twins (DZ) have separate placentas. This means the
prenatal environment of most identical twins is more similar than that of
fraternal twins and so environmental factors could explain the intelligence
correlations. Identical twins sharing a single placenta are more similar in
intelligence than those having separate placentas (Phelps, Davis, &
Schwartz, 1997).
Research evidence against genetic
factors and so for environmental factors
• The correlation of +.28 between adopted children and their biological
mothers found by Horn (1983) is much less than the correlation of
+.42 between parents and children when children aren’t adopted
(Bouchard et al., 1981) and so this difference must be due to
environmental factors.
• Bouchard and McGue (1981) found that the correlation for identical
twins brought up together was +.86 compared to +.72 for identical
twins brought up apart. Thus, whilst twin studies are usually used as
evidence for genetic factors this difference supports environmental
factors because it is due to the fact that identical twins brought up
together have more similar environments than those brought up
apart.
Research evidence against genetic
factors and so for environmental factors
• The Flynn effect shows the environmental factors can have a
substantial effect on intelligence because Flynn (1987) found a rapid
rise in average IQ in many Western countries in recent decades. Such
large and rapid increases in IQ are due mainly to environmental
factors, such as longer time spent in education and greater access to
information.
• Further evidence for environmental factors is provided by Sameroff et
al. (1987) who identified 10 family risk factors related to lower IQ,
which included: mother didn’t go to high school; father had a semiskilled job. They found that at the age of 4, high-risk children were 24
times more likely to have IQs below 85 than low-risk children. On
average, each risk factor reduced the child’s IQ score by 4 points.
EVALUATION OF THE ROLE OF GENETIC AND
ENVIRONMENTAL FACTORS IN INTELLIGENCE TEST
PERFORMANCE
• Sample bias. Identical twins are relatively rare, and identical twins
brought up in separate families are obviously even rarer. (Not enough
data)
• Adoption studies do not isolate genetic factors. Many identical twins
brought up apart were brought up in separate branches of the same
family, and so their environments may have been fairly similar. Other
identical twins were brought up together for several years before
being separated. Note Bouchard’s later study (Bouchard et al., 1990)
addressed this, as it only involved twins separated before 5 months of
age.
• Difficult to interpret the findings from adoption studies. It is very hard to
interpret the findings of many adoption studies because of selective placement,
which is when children are placed in homes similar to those of their biological
parents’ in educational and social backgrounds. Thus, the correlation between
adopted children and their biological parents may be due to selective placement
rather than to genetic factors. (Similar environment)
• Impossible to establish that environmental risk factors cause lower intelligence.
Sameroff et al.’s (1987) findings do not show that the environmental risk factors
they identified were actually responsible for low IQs. It is likely that the parents of
the high-risk children were less intelligent than those of the low-risk children and
so there are differences in genetic potential between the low-risk and high-risk
groups of children. It seems likely that the adverse environmental factors have
some negative effects on children’s intelligence but it is, as always, impossible to
separate out the influence of genes versus environment. (separate genes and
environment)
• Strong empirical support. Twin studies provide convincing evidence
because they allow us to observe the effects of varying degrees of
genetic similarity on intelligence, and so provide strong support for
both the influence of genetic and environmental factors.
• Genetic and environmental factors are positively correlated.
Individuals with the greatest genetic potential for intelligence tend to
find themselves in environments favourable for the development of
intelligence (e.g. staying at school until the age of 18, going to
university). This makes it hard to disentangle the effects of genetic
and environmental factors.
• Correlational evidence. Research into environmental factors is
correlational because the environment cannot be manipulated. This
means cause and effect cannot be established and so we cannot
conclude that environmental factors cause changes in intelligence.
• Validity of IQ tests. Intelligence tests are not necessarily a valid
measure of intelligence. They are culture biased and narrow in scope
because they fail to assess social or emotional intelligence.
Consequently, the evidence is on differences in intelligence as
assessed by intelligence tests, which is not necessarily a valid
measure of intelligence.
Culture
• Culture has an effect on intelligence test performance because the
cognitive skills that are important vary from one culture to another. Thus,
for example, language skills including reading and writing are very
important within most Western cultures, but more practical skills are
emphasised in other cultures.
Culture is an issue in terms of IQ tests because the tests have been devised
by psychologists working in the United States or in Europe. It has
sometimes been claimed that American and/or Europeans are more
intelligent than people from most other parts of the world. However, this is
simply not true. The problem is that the IQ tests are ethnocentric, i.e.
biased to favour the culture in which they were devised, and so they are
not a valid measure of intelligence in other cultures.
Research evidence of cultural differences
• Okagaki and Sternberg (1993) studied ethnic groups in San Jose, California
and found concepts of intelligence varied within a culture. Asian parents
emphasised the importance of cognitive skills in their conception of
intelligence. In contrast, Latino parents argued that social-competence
skills are of particular importance in their conception of intelligence.
• Grigorenko et al. (2004) studied different aspects of intelligence in Yup’ik
Eskimo children living in southwest Alaska. Some of these children lived in
the towns and others lived out in the country. The study tested children’s
practical intelligence (e.g. knowledge of how to travel in the virtual
absence of landmarks) and also used traditional intelligence tests. The
urban children performed better than the rural children on traditional
intelligence tests, whereas the rural children outperformed the urban ones
on the test of practical intelligence. These findings clearly represent the
skills most relevant to the children in their everyday lives.
• Sternberg et al. (2002) argued that children in many cultures perform
poorly on conventional intelligence tests because they have little
experience of this form of assessment. He introduced, where the
individuals are tested on two separate occasions with training in the
skills assessed by the tests being provided between tests. Children in
Tanzania showed substantial improvements between the first and
second test suggesting that they had abilities and an ability to learn
not revealed on the first testing occasion. Thus, dynamic testing in
developing countries can provide a better assessment of intelligence
than traditional single testing.
EVALUATION OF THE RESEARCH INTO
CULTURAL DIFFERENCES
• Validity of intelligence tests. If we want to obtain a valid assessment
of intelligence in any given culture we must always consider the
cultural context. Research such as Grigorenko et al.’s, which has done
this with the assessment of practical knowledge, provides a much
more valid measure of intelligence.
• Dynamic testing is less biased. Dynamic testing is potentially a very
useful way of assessing intelligence. It assesses an individual’s speed
of learning, which is of great importance to intelligence.
Conclusion
• The mutant gene that led to bipedalism may be the origin, and so the
development of intelligence needs to be traced back to this. The co-evolution of
brain size and mental abilities may be in part due to a chance mutation. There is
evidence to support co-evolution as brain size and intelligence are related.
However, other factors are involved in the association. The human brain is highly
complex and more organised than any other animal’s and this may be equally, if
not more, relevant than total brain size.
It seems likely that environmental and then social complexity are the earliest
origins of human intelligence, however, these do not account for why the human
brain is more advanced, at least according to EQ measures! Thus, social and
ecological factors must interact with later factors, more specific to humans, such
as sexual selection, language, bipedalism, and the ability to control fire.
Conclusion
• The environmental factors that influence intelligence testing show
that intelligence is not solely an evolved mechanism. Research
evidence from twin, adoption, and family studies does show the role
of genetic factors, however these studies can also be turned around
to show the influence of environmental factors, for example the fact
that intelligence correlations are lower for identical twins raised apart
than those together is due to environmental factors. The influence of
culture on intelligence shows quite how pronounced the effect of the
environment can be as the very concept of what intelligence is
depends on the cultural context.
Test Essay
• Discuss evolutionary factors involved in the development of human
intelligence.
• (9 marks + 16 marks)
Answer
AO1 = 9 marks Outline of evolutionary factors involved in the
development of human intelligence Factors involved in the evolution of
human intelligence include ecological (eg bipedalism, foraging,
hunting), social (group size, social complexity) and brain size. The
question is on the development of intelligence, so material on, for
instance, the role of diet in increasing brain size must be linked into the
development of intelligence to earn marks beyond Basic. Similarly,
comparative studies of brain size and the encephalisation quotient
need to be presented in the context of the increase in human
intelligence to earn marks beyond Basic.
• AO2/AO3 = 16 marks Commentary on the evolution of human intelligence Research
findings on the evolution of human intelligence are necessarily indirect. However the
research eg of Dunbar on group size, brain size, and social intelligence in human and
nonhuman primate societies, would be an excellent source of AO2/AO3 marks. Research
into gender differences in types of intelligence and cognitive skills have also been
discussed in an evolutionary context and would be creditworthy in this question.
Comparative studies of animal intelligence in relation to, for instance, brain size, may also
be made relevant. Again, however, the focus of the question is on the development of
intelligence, not brain size, and implications for the question must be clear for marks to
be awarded. Although unlikely, candidates may introduce research into the genetic basis
of IQ. This could in theory be made relevant to the question, but this must be explicit for
marks to be awarded. General commentary could include the adaptive advantage of
intelligence (eg in the development of cognitive skills involved in tool use, hunting,
foraging and group living). Additional issues Psychology A (PSYA3) - AQA GCE Mark
Scheme 2011 June series 21 include the problems of conducting research in this area,
the speculative and retrospective nature of many hypotheses, and problems of defining
intelligence. Indicative issues/debates/approaches in the context of the evolution of
human intelligence: evolutionary/biological approach; use of non-human animals in
research; reductionism; free will/determinism; nature/nurture; cross-cultural research;
social/cultural factors; gender differences. Such material must be used effectively to earn
AO2/AO3 credit. AO2/AO3 material should first be placed in the appropriate band
according to the descriptors. However, not all the criteria need be satisfied for an answer
to be placed in a particular band. Weak performance in one area may be compensated
for by strong performance in others. In order to access the top band, issues, debates
and/or approaches need to be addressed effectively.