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.