Were people so afraid to appear bigoted that they lied to pollsters, thinking it more socially acceptable to support a black candidate? Pollsters and political scientists have long questioned that assumption because they do not believe people have an incentive to deceive unless they are explicitly asked, “Do you support the white guy or the black guy?”

“We have no evidence that people lie to us,” said Joe Lenski, executive vice president of Edison Media Research, which conducts the exit polls the television networks use. He and others say that discrepancy in the polls has more to do with which people decline to participate, or say they are undecided.

Adam Berinsky, a political scientist at M.I.T. who has written about the “I don’t know” voters, points out that while polls overpredicted Mr. Dinkins’s support in 1989, they got it right in 1993, when he was running against the same opponent, Rudolph Giuliani. In 1989, Mr. Berinsky argues, people who feared being thought racist said “I don’t know.” By 1993, they could find things in Mr. Dinkins’s mayoral record to object to and so felt more free to express their opposition without fear of seeming racist.

Mr. Kohut conducted a study in 1997 looking at differences between people who readily agreed to be polled and those who agreed only after one or more callbacks. Reluctant participants were significantly more likely to have negative attitudes toward blacks  15 percent said they had a “very favorable” attitude toward them, as opposed to 24 percent of the ready respondents. “The kinds of people suspicious of surveys are also more intolerant,” Mr. Kohut said.

Scott Keeter, Pew’s director of survey research, said pollsters had a harder time reaching voters with lower levels of education. Less-educated whites are the kind Mr. Obama has had trouble winning over. Conversely, young people are more likely to answer surveys, and they tend to favor Mr. Obama.

There may be several factors at work: Michael Traugott, a University of Michigan professor who studies polling, argues that the Bradley effect was misnamed from the start; the problem with the polls in the 1982 race was not that they failed to capture latent racism but that they failed to account for the absentee ballots, which ultimately handed the election to the white Republican, George Deukmejian.

Whatever its causes, the Bradley gap seems to be disappearing.

In a new study, Daniel J. Hopkins, a postdoctoral fellow at Harvard, considered 133 elections between 1989 and 2006 and found that blacks running for office before 1996 suffered a median Bradley effect of 3 percentage points. Blacks running after 1996, however, performed about 3 percentage points better than their polls predicted. Mr. Hopkins argues that the changes in the welfare laws in 1996 and the decline of violent crime took off the table issues that had aggravated racial animosity.