Optimism Bias

What is optimism bias?

Optimism bias, also called optimistic bias, is an expectation that is more favorable than an appropriate comparison warrants. It can concern one's absolute chances of an event or one's chances compared with other people.

In Weinstein’s study (1980), students as a group judged their prospects more favorably than those of peers for many future events. This comparative pattern does not establish that every participant misjudged their own actual probability.

Optimism bias example

Imagine a project team predicting that its launch will finish on time while dismissing delays that have repeatedly affected comparable projects. If the forecast is more favorable than the available evidence warrants, it illustrates optimism bias. A confident forecast supported by relevant evidence would not, by itself, show the bias.

Optimism bias versus optimism

Hopeful expectations are not automatically biased: a relevant benchmark is needed.

Pessimism bias concerns expectations that are less favorable than the evidence warrants.