But what if confirmation bias isn’t the only culprit? It recently struck us that confirmation bias is often conflated with “telling people what they want to hear,” which is actually a distinct phenomenon known as desirability bias, or the tendency to credit information you want to believe. Though there is a clear difference between what you believe and what you want to believe — a pessimist may expect the worst but hope for the best — when it comes to political beliefs, they are frequently aligned.

For example, gun-control advocates who believe stricter firearms laws will reduce gun-related homicides usually also want to believe that such laws will reduce gun-related homicides. If those advocates decline to revise their beliefs in the face of evidence to the contrary, it can be hard to tell which bias is at work.

So we decided to conduct an experiment that would isolate these biases. This way, we could see whether a reluctance to revise political beliefs was a result of confirmation bias or desirability bias (or both). Our experiment capitalized on the fact that one month before the 2016 presidential election there was a profusion of close polling results concerning Donald Trump and Hillary Clinton.

We asked 900 United States residents which candidate they wanted to win the election, and which candidate they believed was most likely to win. Respondents fell into two groups. In one group were those who believed the candidate they wanted to win was also most likely to win (for example, the Clinton supporter who believed Mrs. Clinton would win). In the other group were those who believed the candidate they wanted to win was not the candidate most likely to win (for example, the Trump supporter who believed Mrs. Clinton would win). Each person in the study then read about recent polling results emphasizing either that Mrs. Clinton or Mr. Trump was more likely to win.