We all would like to think we're great, but this desire can get us into trouble. In fact, the brain is extremely good at convincing us, despite all the evidence, that we have lots of positive attributes. A new study in PNAS finds that people who have cheated on a test predict that they will do equally well on a future test in which know they cannot cheat, even if this prediction is costly.

First, the seventy-six participants—all MIT students—were asked to take an eight-question math test, then score themselves and report their performance. Half of the test-takers were given the opportunity to cheat by having access to the answer key on the bottom of the test. Then, both groups were asked to predict their performance on a second math test, with 100 questions and no answer key.

The students that had the answers did better on the first test than those without the solutions, correctly answering an average of one more question. Despite knowing that they wouldn’t have the answers on the next test, they predicted that they would perform better on the second test than the control group did. Compared to the control group, who expected to answer about 73 questions correctly, the cheating group expected to answer 81 correctly.

Even though they knew they had access to the answers, they did not correct for this knowledge when predicting how well they would fare on another test. As expected, they didn’t actually do any better than the control group when given the second test.

But what happens when this kind of self-deception is costly? The researchers offered a reward of up to $20 to a new group of participants, with the reward depending on how accurate their prediction was for their performance on the second test. In this scenario, the participants should be very careful to accurately predict how well they will do if they want to win the most money.

Again, the group with the answers on the first test performed better, and they over-estimated how well they would do on the second test. (Both groups performed equally well on the second test.) Due to their poor predictions, the group that had seen the answers earned significantly less in the experiment than the control group did ($14.47, compared to $17.75). Even when money is on the line, people cannot control this type of self-deception.

The researchers found one factor that makes this phenomenon even worse: social recognition. After taking the first test, a group of randomly assigned participants were given certificates and told that they scored "higher than average." Those participants that had seen the answers and gotten a certificate overestimated their future performance more than all other groups. Social recognition—here, in the form of a certificate—intensifies self-deception.

When it comes to beliefs about ourselves, our strengths, and our weaknesses, we are very prone to bias. Psychologists have proposed that self-deception arose as a way to make it harder to detect lies; if the liar actually believes what he is saying, the lie becomes harder to detect. This may be one of the reasons that self-deception is so embedded in our behavior, and so difficult to prevent.

PNAS, 2010. DOI: 10.1073/pnas.1010658108 (About DOIs).