Bradley effect

The term Bradley effect or Wilder effect refers to a phenomenon which has led to inaccurate voter opinion polls in some American political campaigns between a white candidate and a non-white candidate. 
 
Specifically, there have been instances in which statistically significant numbers of white voters tell pollsters in advance of an election that they are either genuinely undecided, or likely to vote for the non-white candidate, but those voters exhibit a different behavior when actually casting their ballots. White voters who said that they were undecided break in statistically large numbers toward the white candidate, and many of the white voters who said that they were likely to vote for the non-white candidate ultimately cast their ballot for the white candidate. This reluctance to give accurate polling answers has sometimes extended to post-election exit polls as well.

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