Hindsight bias is a documented psychological phenomenon in which people exaggerate the predictability of an event after it has already happened. Some psychologists refer to this phenomenon as the “I knew that was going to happen” effect. According to a study performed by the American Psychological Association in 2000, this bias actually helps people think more clearly sometimes, by helping the brain to retain correct and relevant information rather than incorrect information.
You can probably think of a few examples of hindsight bias in your life, especially if phrases like “hindsight is also 20/20” and “I told you so!” sound familiar to you. This bias works in a number of ways, and it is especially important to take it into account in criminal cases, because a witness may not be strictly accurate, since he or she may be influenced by hindsight bias, along with a number of other biases which can influence the way someone's brain restores and recalls information.
A classic example of this phenomenon occurs when someone claims that his or her prediction about an event was more significant that it really was. For example, someone might generally observe that “it looks like rain in the future,” given his or her general knowledge of local weather patterns. If it rains shortly after this statement is made, the person might feel that the prediction was stronger than it really was. Incorrect or inaccurate predictions tend not to be remembered as well as vaguely correct predictions, reinforcing the idea in someone's mind that his or her predictive skills are better than they really are.
In a specific phenomenon called vaticinium ex eventu, someone makes an extremely vague statement about an event which might occur, and then turns that statement into a solid prediction after the event has occurred. This is sometimes scathingly called “postdiction.” Many examples of vague predictions which were later thought to be more important than they actually were can be found in Greek mythology, where the cryptic oracles make blanket statements which could easily be said to be predictive of a great number of events.
Along with several other biases documented in psychology, the hindsight bias is caused by something known as an availability heuristic. Essentially, people make assessments about things on the basis of information which they can bring readily to mind, although this may not be the most scientific way to base such an assessment. For example, someone might visit a particular franchise in a fast food chain and note that all of the patrons there are overweight. He or she might then say that all patrons of that chain are overweight, on the basis of this single example. In the case of hindsight bias, people turn a few vague statements into solid predictions, and assume that an event like the outcome of a Presidential election is predictable on the basis of their experiences.