Optimism bias is a cognitive bias assumed to cause a person to believe that they are at less risk of experiencing a negative event compared to other people. An example of the optimism bias is when smokers believe that they are less likely than other smokers to contract lung cancer by smoking. Optimism bias is usually measured through determinants of risk. Individuals are asked to estimate the likelihood that they will experience a negative event compared to the likelihood that another person will experience the same negative event. The bias is demonstrated by data showing that the mean risk estimate of individuals estimating their own risk is lower than the mean risk estimate given to others.

Source:

Weinstein, Neil D.; William M. Klein (1995). “Resistance of Personal Risk Perceptions to Debiasing Interventions”. Health Psychology 14 (2): 132–140.