How we change what others think, feel, believe and do
If we believe that someone else is in a group to which we belong, we will have positive views of them and give them preferential treatment.
This works because we build our self-esteem through belonging, and the presence of someone from an in-group reminds us of that belonging.
The opposite of in-group bias is out-group bias where, by inference, out-group people are viewed more negatively and given worse treatment. This is the basis of racial inequality.
In-group linguistic bias is where out-group people are described in abstract terms (which depersonifies them) when they conform to the out-group stereotype. Out-group people will be referred to in more specific, concrete terms when they act in unexpected ways.
Henri Tajfel visibly divided people in to random groups. They rapidly found in-group people preferable to out-group people, even finding rational arguments about how unpleasant and immoral the out-group people were.
Watch children in the school yard. Notice how they form groups and how they treat those not in their gang.
Make yourself and the other person a part of the same group, and they will be biased towards you (and away from anyone you cast as out-group).