Suddenly, high school horseplay has become a matter of national importance.

What prompted this renewed debate on pranks and youthful bad behavior? Presumptive Republican presidential nominee Mitt Romney stands accused of bullying a gay classmate decades ago at the private high school he attended, apparently going so far as to forcibly cut a fellow student’s long, bleached-blond hair as others held the boy down, according to the Washington Post.

While Romney says he has no memory of such an incident, he issued a general apology for any pranks he took part in during his youth: “Back in high school, I did some dumb things, and if anybody was hurt by that or offended, obviously, I apologize for that…. I participated in a lot of high jinks and pranks during high school, and some might have gone too far, and for that I apologize.”

But how can you tell when a prank — like waxing someone’s eyebrows while they sleep or punking a germaphobe coworker — has gone too far and strayed into bullying territory? Are high jinks and teasing just a natural, innocuous part of life, or can poking fun at others do real, lasting harm?

There’s evidence that teasing actually can be helpful in some cases. University of California at Berkeley psychologist Dacher Keltner has been studying the topic for years, and he’s found that people have long used teasing to establish group morals and social hierarchies, build and test relationship bonds and safely convey challenging concepts and emotions — all theoretically good things.

In one experiment, Keltner and his collaborator Erin Heerey invited fraternity brothers and their pledges to their lab and had them tease one another. They found that while the frat brothers’ teasing of the pledges was at times quite pointed, all of them became better friends because of the playful back and forth. In fact, the more the target of the tease showed signs of embarrassment — blushing, averting his gaze, smiling nervously — the more the teasers ended up liking him.

But there’s a difference between light-hearted teasing that’s meant to bring people closer and the type of bullying, pranks and jokes that highlight social differences.

“Jokes, like pranks, accentuate the differences between the jokers and those whom the joke is on.”

“Jokes, like pranks, accentuate the differences between the jokers and those whom the joke is on,” Moira Smith, an anthropology librarian at Indiana University, told Wired by phones. Smith has spent several decades researching practical jokes, starting with elaborate “capping stunts,” or student pranks, she catalogued in her native New Zealand. (One time, her academic colleagues in Wellington pretended to be health ministers and convinced folks all over the city to drop off urine samples at the central post office.)

She’s also researched historical pranks like the Berners Street Hoax, a sort of primordial flash mob orchestrated in 1809 by renowned British practical joker Theodore Hook. The prankster sent thousands of fictitious letters to people all over London, convincing a small army of them — chimney sweeps, fishmongers, cake bakers, vicars, even the Duke of York and the Lord Mayor — to appear at the same date and time at the home of a very baffled woman named Mrs. Tottenham.

While such stunts might sound like fun, Smith argues they’re anything but if you happen to be the target of them. That’s because pranks and other forms of disparaging humor — racist jokes, sexist jokes, derogatory jokes — are all about hammering home that you’re not part of the group. And since such attacks are couched within the confines of comedy, they can be harsher and more insulting than would otherwise be allowed in polite society.

There’s another problem with bullying and pranks, one that’s downright troubling. Disparaging humor doesn’t just highlight social divisions; it actually has the potential to further the divide, according to Thomas Ford, a psychology professor at Western Carolina University. Ford developed the “prejudiced norm theory” (.pdf), the idea that disparaging jokes can increase tolerance of discrimination.

In one experiment, Ford asked a bunch of undergraduate males to watch a variety of comedy videos. Then he gave them what they thought was a real assignment: Cut funding for different student groups such as a study-abroad club, a Jewish organization, a black student union and a women’s council. Not all that surprisingly, the students who were gung-ho about slashing the funding for the women’s group where among those who’d previously scored high for hostile sexism.

But here’s where things get interesting: Among all the men who rated high for hostile sexism, only those who’d first seen funny videos degrading women, such as a skit from The Man Show about sending annoying spouses to “wife school,” were willing to slight the women’s group. The similarly sexist guys who had had instead watched an innocuous clip, such as one of the E-Trade talking-baby commercials, were no more willing to downsize the women’s group than those who didn’t appear to be sexist at all.

The limit of what society deems acceptable is like a rubber band, Ford told Wired by phones. Derogatory jokes, by letting people goof around with taboo subjects in a noncritical manner, tend to stretch the band of acceptability into areas hitherto off-limits — and once it’s stretched, it’s hard to go back.

That brings us back to Romney’s “Bullygate” scandal. Were his immature high jinks designed to build bridges, or were they about separating the haves from the have-nots? The purported hair-cutting incident — said to have left the victim in tears, and some of the participants shaken — sounds like the latter. It sounds like the kind of roughhousing that goes too far and does lasting harm — not just to the target of the prank, but also to the pranksters.

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Professor Peter McGraw and writer Joel Warner have teamed up to explore the science of humor on a global expedition. The Humor Code chronicles their adventures, scientific experiments and unintentional comedy along the way. Learn more about McGraw, Warner and their escapades at HumorCode.com.