Can You Trust the Polling on Proposition 8?

Last week's Field Poll in California found that voters were likely to reject the state ballot amendment in November that intends to prohibit gay marriage. But are voters really telling pollsters the truth about how they'll vote once they're in the booth? A polling expert examines the numbers and the opposition's claim that the Field Poll just isn't accurate.

BY Editors

September 23 2008 12:00 AM ET

PROP. 8 x100 (Getty) |

Though’s methodology was flawed, the phenomenon of voters underreporting their biases -- often referred to as “the Bradley effect” -- does have a historical basis. It takes its name from Tom Bradley, the African-American former mayor of Los Angeles who staged an unsuccessful run for governor of California in 1982. Two Field Polls conducted shortly before the 1982 election found Bradley with a significant lead over his white Republican opponent, George Deukmejian. But ultimately, Bradley ended up losing by just one percentage point. Analysts assume that one reason for the discrepancy between the polls and the election result was that white supporters of Deukmejian were reluctant to appear intolerant to survey researchers and thus falsely reported being undecided. Other polls have similarly underpredicted the performance of white candidates running against African-Americans in other high-profile races, including the 1989 contest for Virginia governor and the '89 and '93 New York City mayoral elections.

But recent research has suggested that the “Bradley effect” has subsided. A paper by Harvard University’s Daniel Hopkins analyzing state-level elections since 1989 estimates that the size of the Bradley effect -- which was never very big to begin with -- is now effectively zero. Similarly, no effect is found for female candidates. And despite much hand-wringing in the media of a Bradley effect hurting Barack Obama, he actually outperformed preelection polls on average by about three percentage points during the 2008 presidential primaries.

My analysis indicates that it is unlikely that recent polling on Prop. 8 in California substantially understates support for the initiative. Given how much the marriage ban is currently trailing in the polls, the probability is very low that a Bradley effect would lead to a Prop. 8 victory if the election were held today. Stay tuned, however: Both sides have raised millions to finance what will undoubtedly be a knock-down, drag-out fight via TV ads. A lot can change in seven weeks -- including polls. But when new surveys are released, take voters at their word: They are telling the truth.