American Gridlock: Chapter 16
Congressional Polarization and Its Connection to
Income Inequality: An Update
Adam Bonica, Nolan McCarty, Keith T. Poole, and Howard Rosenthal
Summary: Editors
• 1. Polarization in Congress is the highest since
Reconstruction
• 2. Polarization is not an artifact of roll call voting. It also
occurs in campaign contributions.
• 3. Polarization in Congress is largely due to the Republican
Party becoming more conservative.
• 4. Polarization and income inequality appear to be
mutually causal.
• 1. The Republican Caucus in the House has moved to
the Right with every election since 1976.
• 2. The Left-ward Movement of the Democratic Caucus
is due to the Southern Realignment. Northern
Democrats have been relatively stable since the 1960s on
the first dimension (however, this could change in the
Age of Trump).
• 1. Many Scholars have not read Congress: A PoliticalEconomic History of Roll Call Voting (1997, New York:
Oxford University Press) very carefully.
• 2. Rosenthal and I show that through most of American
history that there is an important second dimension that picks
up regional conflicts that divide the two major parties
internally.
• 3. Instead, many scholars say that “Poole and Rosenthal
(1997) show that voting in Congress is one-dimensional.” Or
that the two-dimensional results are simply “summary
measures of partisanship.”
• 4. So, it is worth looking at the next graph:
• 1. Polarization as measured by the distance between the
two major parties on the first (Liberal-Conservative in
the modern era) dimension is strongly correlated (R =
0.97) Adam Bonica’s CF scores.
• 2. CF scores are based on scaling literally millions of
campaign contributions by PACs and individuals (A
very impressive achievement).
• 1. The Party Means in the House from CF scores and DWNOMINATE scores track very closely.
• 2. The biggest difference is that CF scores do not produce
the same pattern of asymmetric movement. This is most
likely due to the distribution of contributors being more
symmetric. “We conjecture that Democratic Party
activists are moving to the left faster than the
Democratic members of Congress who, in our opinion,
are close to the left edge of acceptable public policy on
economic issues in the United States” (p. 366).
• 1. The next three graphs show House polarization against
the Gini Index of Income Inequality; top 1% Income Share;
and the Income of Individuals in Financial Services relative
to other sectors of the economy.
• 2. Note that in the second and third graphs if House
polarization is lagged 10 years the correlations jump
dramatically.
• 1. The last two slides are from Adam Bonica’s work on
campaign contributions.
• 2. The first shows that in 2012 the mega-Rich accounted
for more than 40% of the contributions!
• 3. The last slide shows the ideological distribution of
contributors broken down by billionaires, the top 0.01% of
the income distribution, the Forbes 400/Fortune 500, and
small donors.
• 4. Note that small donors are the most polarized!
Figure 5: Concentration of income and campaign contributions in the top 0.01% of
households and voting age population
Share (in %) of Income/Contributions
40%
30%
20%
10%
0%
1980 1982 1984 1986 1988 1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012
Year/Election Cycle
Campaign Contributions
Income