Many studies have found an effect in which people predict that positive items are more likely to occur to them than negative items. A classic study by Rosenhan and Messick (1966) provided people with drawings of people with smiling faces or frowning faces. In one condition, people had a set of pictures that were made up of 70 percent smiling faces (thus, 30 percent were frowning), while the other condition had 70 percent frowning faces. The participants were asked to predict whether a smiling or frowning face would be shown next. People in the 70 percent smiling faces condition were very accurate in their predictions in that they predicted 68 percent to be smiling. People in the 70 percent frowning condition were much less accurate in that they predicted a frowning face only 57 percent of the time (much less than the actual 70 percent). Thus, it seems that the people in this study were biased to predict positive events as being more likely (or negative events as being less likely). That is, they may have thought that they were more likely to have a smiling face shown to them than a frowning face.

Other studies have found similar effects in predicting life events. Weinstein (1980) asked college students to predict how likely positive and negative events will occur in their lifetime as compared to their fellow classmates. These students predicted that they would be 42 more likely to have a good starting salary after graduation and 38 percent less likely to have a heart attack. Thus, the students in this sample thought they were more likely to have better outcomes and less likely to suffer bad outcomes than their fellow students.