People are said to be unrealistically optimistic when they underestimate the chances of experiencing a negative event or overestimate the chances of experiencing a positive event. This bias was first defined by Neil Weinstein as a social comparison with others, namely, the mistaken belief that we are less likely than others to experience negative events and more likely to experience positive events. However, unrealistic optimism can also be established by comparing a prediction with an actual outcome or with a more objective prediction of that outcome. Other terms representing this bias are optimistic bias and illusion of unique invulnerability. Unrealistic optimism is one of several biases related to the self-concept, such as the better-than-average heuristic (whereby people believe they are better than others on a wide range of dimensions), the uniqueness bias (the belief that one’s abilities and moral attributes are unique), and the self-serving bias (thinking that one’s successes are due to internal causes and failures are due to external causes), and these various biases have similar causes, as enumerated in what follows.