Faced With Evidence, We Still Get It Wrong

March 9, 2011— -- So you want your political party to take control of the state legislature, and you've just learned that the largest newspaper in the state has thrown its support behind your favorite candidate. Will that make you more or less likely to believe your party will succeed?

It may seem counterintuitive, but the answer is less likely, according to a new study from Brown University in Providence, R.I.

Why would evidence of growing support for your party make you less optimistic?

Because it's weak evidence, according to cognitive scientist Philip Fernbach, professor of psychology at Brown and lead author of a study in the journal Cognition.

Obviously, one endorsement in a political campaign with many candidates for many seats is not going to swing the entire election in one direction. The endorsement may be positive evidence, but it's not conclusive, and when that's the case, according to the researchers, the evidence may backfire.

In five experiments involving participants across the country, people who were given weak evidence in support of a possible cause (more troops should lead to stability in Afghanistan, for example) were less likely to believe the war-torn country would eventually see stable days than people who were given no evidence at all.

Fernbach and two colleagues, Adam Darlow and Steven Sloman, say their research shows that weak evidence can cause someone to make a bad decision, because humans tend to be focused on whatever they have just learned or experienced.

"It turns out that if you give people some evidence that is positive but weak, then actually they focus too much on that piece of positive evidence" and are less likely to incorporate other facts in their decision, Fernbach said in a telephone interview. In all five experiments, participants were required to make a quick decision, so it was an "intuitive judgment that happens quickly, like when you are shopping."

But weak evidence probably influences long-term decisions as well, he added, and undoubtedly plays a role in everything from shopping to electing a president.