The Double Edged Passion

Humans are passionate creatures. Our passions drive us, gives us a sense of belonging, and unite us as few other things can. Still, there are only a couple of passions that have been constants down the ages, passions that people from every place and culture can agree on. Love is one, but another is that "those no-good bastards over there are trouble." Of course, we quibble endlessly over the exact definition of "those" -- every culture, pretty much, has had a different group in mind. But the singular fact of prejudice per se was as recognizable in Ancient Greece, Rome, and Samaria as it is now in modern Greece, Rome, and Arkansas.

Not only do we disagree over who Those Bastards Over There (TBOT) are, but also over why we hate them so much. Nose size has been cited in the past as a reason, as has intellectual capacity (too much or too little), and bad manners (eating without implements, eating with implements, etc). Hate may be a massively universal thing, but we are shockingly divided over why we do it. Personally, I blame TBOT.

Psychologists, though, (hate us or loathe us) aren’t as sanguine about not knowing, and have spent a great deal of time investigating  prejudice in its many guises. They have come to two broad classes of answers: (a) Reasons we hate each other, and (b) Reasons we think we hate each other. There's not as much overlap between those two as you might hope.

Why We Hate - A First Stab at Them It.

Scientific psychology started getting seriously interested in prejudice just after the Second World War. There's nothing like a conspicuous mountain of corpses to really get you going on the question of hate.
The first really influential answer that psychology came to was not the ever popular theory that "some people are just jerks," but neither was it far off. A German intellectual called Theodor W. Adorno released a book in 1950 called "The Authoritarian Personality," in which he detailed research on his "F-scale." This scale was designed to pick out people who were, among other things, conventional minded, uncritically accepting of authority, and accepting of the need for authorities to aggressively apply their power. He called it the F-scale, because he thought it would pick out people prone to fascism. Other people pointed out that it might not do a bad job picking out Soviet style communists either, but as a committed Marxist Adorno wasn't as taken with this application.
These ideas have been updated as the construct of “Right Wing Authoritarianism,” about which the leading authority, Bob Altemeyer, has written a highly readable book which is available for free online HERE. New research (e.g. Jost, 2006) is adding to this showing that people who are very low in the commonly measured “openness to experience” construct seem to be more likely to be both right wing, and prejudiced.

Psychologists have long noted people's over-fondness (at least in the western world) for explaining actions in terms of the personalities of the actors involved. We tend to neglect the possibility that a person who falls, for example, might have been tripped, and that they aren't just clumsy. This bias is so commonplace that psychologists have named it the  fundamental attribution error. Perhaps it isn't surprising, then, that the first attempt to explain prejudice chalked it up to an authoritarian personality. If we want to get past this known bias we must ask, then, might hate also be caused by one’s circumstances, and not always just by an ornery disposition?

Why We Hate - Situations That Lead to Hate

In 1954, psychologist Muzafer Sherif spent a summer dressed as a janitor at a summer camp to which he had brought two dozen perfectly normal 12 year old boys. In this classic study the boys were divided into two separately housed groups, who spontaneously took on names for themselves - the Eagles and the Rattlers.
There, he tried out one of the oldest, gold plated, tried and true, best ways to get people to hate each other: finding something they both can't have. Historically speaking, jobs, land, money, churches, all have been favourites. Sports leagues have used this principle for years with silver cups. Psychologists call it the  realistic threat hypothesis. A quick glance at those sports leagues will show that the prize doesn't have to be realistically valuable, though, just somehow real. Still, honor and prestige are both real to human minds, as is “truth,” that most hard-fought-over piece of mental real estate. Get people excited about any of the above, and you’re well positioned for a launch down the well-trodden road to belligerent disrespect.