Social Desirability Bias in Polling on Same-Sex Marriage Ballot Measures

Prior public opinion research has identified a wide range of circumstances in which polling results may be tainted by social desirability bias. In races pitting a Black candidate against White opponents, this has often been referred to as the “Bradley effect” (aka “Wilder effect” or “Dinkins effect”), by which survey respondents overstate their preference for Black candidates running against White opponents. This study examines the accuracy of polling on same-sex marriage ballot measures relative to polling on other statewide ballot issues in all states voting on the issue from 1998 to 2012, controlling for a range of theoretically relevant contextual factors. There has been a great deal of speculation, though little empirical evidence, that polling systematically understates opposition to same-sex marriage. Consistent with social desirability bias, this study finds that opposition to same-sex marriage is about 5% to 7% greater on election day than in preelection polls.