Humans are prejudiced. Some more than others, but it would be difficult to find an individual who shows no biases at all. Prejudice is a positive or negative attitude towards a social group, typically based on judgments about race, gender, age, ethnicity, or religion. Prejudice can also be based on diverging ideologies or political perspectives, on different accents, different body shapes, sexual orientation, nationality, location, income, etc. Both in the past and today, we find prejudice all over the world. Most importantly, much prejudice and bias is unconscious or implicit - we don't know just how prejudiced we all are.

So why are people prejudiced? There are many contributing factors, but one key factor is that humans tend to form groups: in-groups and out-groups. Indeed, one could argue that prejudice only exists because of categorisation. If we didn't form groups or see differences, there would be no prejudice. Prejudice may also have had evolutionary advantages. Being distrustful of other groups could have enhanced in-group co-operation, and protected that group from exploitation, free riding or aggressive capture of resources through inappropriate trust.

A second reason for prejudice is the operation of stereotypes: perceived knowledge about a social group. Gordon Allport stated that stereotypes might have a “kernel of truth”, but one which is often misinterpreted, or over-extended, leading to the false belief that we are able to predict an individual's behaviours based on one piece of information, such as ethnicity or religion. However, prediction about an individual's future behaviour statistically increases in accuracy when multiple factors are taken into account, suggesting that individuals who use stereotypes may get a simple but inaccurate prediction.