by Anthony G. Greenwald, professor of psychology, University of Washington
and Bethany Albertson, assistant professor of political science, University of Washington
Revised March 14, 20081
In analyzing polling data as the primaries and caucuses progress, we have found that race still plays a role in American politics but that it showed up in surprising ways in the tallies from some of the states holding Democratic primary elections so far this year. The so-called “Bradley effect” was first noticed by survey researchers in 1982 when black Los Angeles Mayor Tom Bradley had a solid lead in the pre-election gubernatorial polls, but lost a close election in California to his Republican opponent. Results from that and other races involving black candidates indicated that, for whatever reason, pre-election polling tended to overstate support for black candidates compared with their actual vote percentages.
Throughout this year’s primary season, we have been comparing data from pre-election polls with actual voting patterns as revealed in exit polls to see if the Bradley effect is still operative. In research we jointly undertook last December, we analyzed data from an online test that measures unconscious or automatic preferences.2 On the basis of our findings, we surmised that the Bradley effect might well repeat itself in 2008. Our more recent findings, however, suggest a more complicated pattern.
Analysis of primary counts and polling data from the early primaries, including those held before and on Super Tuesday (February 5), indicated that pre-election polls did indeed exaggerate support for Sen. Barack Obama in three states with relatively low black populations — New Hampshire, California and Massachusetts. But the reverse was true in South Carolina, Alabama and Georgia, where blacks make up a larger bloc of voters.
As shown in the graph, the findings in South Carolina, Alabama and Georgia suggested to us the discovery of a new “reverse” Bradley effect, i.e., that in states with relatively large African American populations, pre-primary polls tended to underestimate support for Obama. (View a larger version of the graph)
The strength of these two effects is shown in the fact that the differences between the actual results of the Democratic primaries in California, Massachusetts, New Hampshire, South Carolina, Georgia and Alabama as well as the polling numbers from three subsequent Democratic open primaries substantially exceeded the polls’ expected margin of error, being off by between 8 and 18 percentage points. By contrast, we found that Republican vote totals and poll numbers were off substantially only in Massachusetts where Mitt Romney’s winning margin was less than predicted.
The February 19 results in Wisconsin — where the black population is relatively small but pre-primary polls significantly underpredicted Obama’s large win – did provide a strong exception by presenting a “reverse Bradley” effect in a state with low black population. Results of the subsequent March 4 primaries, however, suggest that Wisconsin was a one-time outlier or that the phenomenon observed there was transitory.
The Bradley effect was strongly evident again in Rhode Island, with its relatively small black population, where pre-election polls underestimated the Clinton-Obama vote gap by more than 8 percentage points. The outcomes in Ohio and Texas were approximately in line with expectations although in both states, Obama’s vote was over-predicted slightly. In retrospect then it appears that Obama’s success in Wisconsin did not mean that the dynamics of race had fundamentally changed by that point in the primary season.
In the most recent primary in Mississippi with its high black population (36%), the results again show a reverse Bradley effect. It’s nevertheless interesting that the “MS” point as shown in the graph, is noticeably above the regression line, although it is far less of an outlier than Wisconsin. Moreover, despite a 24-point loss in the popular vote, Clinton in a sense did “better than expected,” when one generates the expectation from the combination of regression plot and pre-election polls.
The remaining primaries may provide further evidence on the persistence of both the Bradley and reverse Bradley effects. Still, it is evident from the substantial deviations shown in the graph — with the Obama-Clinton gap off by more than 8 percentage points in 9 of 16 open primaries — not only that race is still strongly operative as a factor in America’s state elections, but also that its impact depends in substantial part on the racial mixture of the state in question.
1 Earlier version of this analysis were posted on pewresearch.org on February 7, 2008 and March 5, 2008.
2 More information on the study of hidden biases can be found at the Project Implicit website.