http://en.wikipedia.org/wiki/In-group_favoritism
http://en.wikipedia.org/wiki/Group_conflict
http://en.wikipedia.org/wiki/Out-group_homogeneity
http://en.wikipedia.org/wiki/Common_ingroup_identity
And a couple of interesting experiments:
...that the state is a creation of nature, and that man is by nature a political animal. And he who by nature and not by mere accident is without a state, is either a bad man or above humanity; he is like the "tribeless, lawless, hearthless one," whom Homer denounces—the natural outcast is forthwith a lover of war; he may be compared to an isolated piece at draughts.
Or, of course, the Stanford Prison Experiment: http://www.prisonexp.org/
More subtly: the notion of American "rugged individualism," and the group with which many (right-leaning) "indiviudalists" unironically self-identify.
On HN, I suspect there's high participation the group of rational, logical thinkers who consider themselves superior to the fears and insecurities that compel the typical "joiner."
If this was irony, it was splendidly done.
http://www.ncbi.nlm.nih.gov/pubmed/15491274
> Four experiments confirmed that women's automatic in-group bias is remarkably stronger than men's and investigated explanations for this sex difference, derived from potential sources of implicit attitudes (L. A. Rudman, 2004).
PDF: http://rutgerssocialcognitionlab.weebly.com/uploads/1/3/9/7/...