The Bradley effect, less commonly called the Wilder effect, is a theory proposed to explain observed discrepancies between voter opinion polls and election outcomes in some US government elections where a white candidate and a non-white candidate run against each other. Instead of ascribing the results to flawed methodology on the part of the pollster, the theory proposes that some voters tend to tell pollsters that they are undecided or likely to vote for a black candidate, and yet, on election day, vote for his white opponent. It was named after Tom Bradley, an African-American who lost the 1982 California governor’s race despite being ahead in voter polls going into the elections.
The Bradley effect theorizes that the inaccurate polls were skewed by the phenomenon of social desirability bias. Specifically, some white voters give inaccurate polling responses for fear that, by stating their true preference, they will open themselves to criticism of racial motivation. The reluctance to give accurate polling answers has sometimes extended to post-election exit polls as well. The race of the pollster conducting the interview may factor in to voters’ answers.
Some analysts have dismissed the theory of the Bradley effect, or argued that it may have existed in past elections, but not in more recent ones. Others believe that it is a persistent phenomenon. Similar effects have been posited in other contexts, notably the Shy Tory Factor and spiral of silence.
– From Wikipedia