There are a huge number of references online. Just Google “in group out group social evolution”. Or simply go to Wikipedia for a quick summary: https://en.wiki

There are a huge number of references online. Just Google “in group out group social evolution”. Or simply go to Wikipedia for a quick summary: https://en.wikipedia.org/wiki/In-group_and_out-group https://en.wikipedia.org/wiki/In-group_and_out-group Now I think we are done here.