Partyforumseasia: The great biologist  Edward O.Wilson, (The Social Conquest of Earth, New York 2012) studied ants to better understand human group behavior. The psychological evidence is most interesting for partisanship and party membership:

“Experiments have shown that it is shockingly easy to elicit a sense of solidarity among a group of strangers. Just tell them they’ll be working together as a team, and they immediately start working together as a team, all the while attributing to each other a host of positive qualities like trustworthiness and competence—an instant five-star customer review.
Yet we are equally prepared to do battle against those who fall outside the fraternal frame. In experiments where psychologists divided people into groups of arbitrarily assigned traits—labeling one set the Blue team and another the Green, for example—the groups started sniping at each other and expressing strong prejudices toward their “opponents,” with the Greens insisting the Blues were untrustworthy and unfair. The “drive to form and take deep pleasure from in-group membership easily translates at a higher level into tribalism,” Wilson says, and can spark religious, ethnic and political conflicts of breathtaking brutality.”  Source: Smithsonian Institute

Partyforumseasia: So far so bad, being member of a successful party feels good, but inside political parties tribalism among factions can be as or even more brutal than with outside enemies. A quotation attributed to Konrad Adenauer, first chancellor of post war (West) Germany, should warn us: His line was: enemy – mortal enemy – party comrade…