I'm just reading https://news.yale.edu/2019/03/26/blueprint-evolutionary-origins-good-society. Among other things, physician and Yale sociology professor Christakis points out that three-month-old babies have sense of fairness and reciprocity, and that children will create in-groups and out-groups on the strength of nothing but the color of a group's tee-shirt. "...people are much more likely to be enemies of people they knew than of people they did not know...each ten extra friends being associated with one extra enemy..." many interesting charts, graphs.
I'm just reading https://news.yale.edu/2019/03/26/blueprint-evolutionary-origins-good-society. Among other things, physician and Yale sociology professor Christakis points out that three-month-old babies have sense of fairness and reciprocity, and that children will create in-groups and out-groups on the strength of nothing but the color of a group's tee-shirt. "...people are much more likely to be enemies of people they knew than of people they did not know...each ten extra friends being associated with one extra enemy..." many interesting charts, graphs.