Male-to-male greeting in America takes many forms. There’s the classic handshake. The fist bump. Dap. The head nod. The you-too-huh? shrug from across a baby shower.* But as the world of masculine salutations takes on new layers of complexity—reaching its most evolved form in Cleveland—there is one fixed practice that remains something of a universal truth: When hugging, two (usually) heterosexual men will almost always pat each other on the back.

Start paying attention, and you’ll see it everywhere. We can’t help it, as if it were a particularly pernicious tic or social crutch, like constantly checking your phone during dinner or hitting a vape. And though the most commonly accepted explanation is true—that a not insignificant part of it is born out of the admittedly primitive heterosexual norms that deem tenderness among males not “masculine” (more on that to come)—there must be some deeper anthropological basis for slapping another guy on the back. And, according to experts, there is!

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But first, we need to set the table: Why do we even hug?

“As forests receded, we were no longer forest-dwelling apes but upright hominids on a plain," says Mark Bowden, human-behavior and body-language expert. "We can now see a distance, and so we need clear signals that somebody is a friend or a predator. So open body language and open palms—imagine hands up, that big surrender gesture—somebody can see two miles away that you're not a threat."

This “Look, I’m not going to stab you with a spear” measure is especially important to establish when the hominids happen to own penises.

”Testosterone makes people more risk-tolerant," says Bowden. "So you will get more aggression the more testosterone [there is], not because the testosterone is making somebody more aggressive. What it's doing is lowering the idea of there being a risk in the first place… [So] groups of males, on the whole, [have] a lot of behaviors to countermeasure the possibility of aggression.”

And what's the best behavior to countermeasure the aggression when those two miles become no miles, and you're now faced with that guy you saw in the hazy distance 20-some minutes ago, across the plain? Sure, a handshake might work. But there’s actual value in doing something more intimate to quash any suggestion that you're going to smack him with a cudgel and steal his collection of exotic sabertooth furs, like hugging. Take it from Richard Wrangham, who works in Harvard’s Department of Human Evolutionary Biology (and who e-mail-answered my strange request for comment after six zoos declined).

“There is a general principle involved in animal alliances, such as male-male friendships in primates: If two individuals are to express feelings of mutual solidarity, the reliability of the signal is greater if it is genuinely somewhat stressful. For example, male baboons who like each other but want to be sure about each other's feelings touch each other's genitals: If A can do that to B, and B doesn't snarl back, A can be truly confident that B likes him.

“[This theory] suggests that males would basically prefer not to pat or hug, because such close physical proximity is ultimately somewhat stressful (given that it is potentially dangerous to be so close to someone who could be a secret rival). However, the stress is worth tolerating if it leads to confidence in each other's feelings about each other.”

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All right, so we hug so that we know who the real ones are. And we do it in a very specific way, says Bowden—with open palms around the shoulder blades. The open palm not only indicates the absence of a weapon, but the “flat hand on skin is going to cause levels of oxytocin to go up, which will actually cause more of a connection.” (And the upper back is very well-protected, versus the belly or sensitive small of the back, both of which would make you feel far more uncomfortable and intimate.)

“I think what it's about is two males being able to show vulnerability, but not in so vulnerable a way that if there was attack or real aggression, they'd be in trouble,” says Bowden.

So, why the back pat?

“The pat has that little of physical roughness to it, which is also consistent with men,” says Adam Galinsky, a social psychologist who has a B.A. from Harvard, a master's and a Ph.D. from Princeton, and teaches at Columbia Business School and Kellogg School of Management. “Men wrestle with each other. It has the unique masculine quality of rough play, with the distancing behavior—you're saying, ‘I'm being intimate, but I'm not crossing the line into being too intimate.’” Over time it starts to feel like a "uniquely special male thing," and the hug then becomes a ritual.

But the pat! “The pat can be used for a signal of release,” adds Bowden. “A lot of primates have this tapping out behavior—during play-fighting, that's the I'm done. Let's not move this into the realm of actual grappling. Pat pat pat, and now we're out. Let's not prolong this too long. If you prolong it, there's risk of further intimacy or aggression.”

“Go get your bro and hold him close.”

The pat is part physical foolishness and part signal of an embrace's termination—and it's now fully ingrained in male-greeting liturgy.

However, any form of greeting is not just about the two parties involved. Bowden argues that a gentle pat among friends, both visible and audible (the slight sound of hand-on-back), indicates to the surrounding group—whether that be a bunch of primitive, aggressive cavemen gathered around a carcass on the African plains or a bunch of primitive, aggressive cavemen at a Patriots game—that the newcomer’s hands are empty, and he is benign. Of course, sometimes the newcomer doesn't want to be benign. He will try to manipulate the optics of what should be a harmless exchange into some weird dick-swinging contest, an attempt to signal to the herd who the one true Daddy is. This type of toxic insecurity is also, unfortunately, where the homophobia creeps in.

You’re trying to figure out the “tribal-social norms,” says Bowden. “What is the normality for a heterosexual man to be intimate with another heterosexual male? And how can you make sure that you, as the tribe, don’t overstep those boundaries?” Galinsky refers to these boundaries, too, saying the pat is an “integrative solution” that allows men to hug each other while not doing anything that would make the “tribe” uncomfortable. The need to establish heterosexuality ties back to the play-fighting/grappling aspect also: Look at us! Just a couple of dudes, roughhousing, being guys! And when you really overdo it...well, you just might be overcompensating.

“You could see some extremes of quite big, aggressive play behavior in groups of males that want it very much to be known to themselves and others, Look, there's nothing homosexual going on here," says Bowden. "Now, we could drill into all kinds of reasons why they might want it to be very, very obvious. There's one school of psychology that says they're very unsure. They want to make it very physically clear, because psychologically they're a little bit on the boundary.”

But, guys: It's 2017! Can we really not be tender with one another, without fear of feeling emasculated or castigated? Maybe it's time we update our tribal norms. I love a hearty back pat as much as the next guy, and if it's to signal the end of an embrace or a means of physical buffoonery among friends, that's cool. But if it's because you're afraid of a little physical affection? Leave that type of limited thinking to the monkeys, man. Go get your bro and hold him close.

"Once we become aware of this, it actually helps us understand where we stand with people or how we feel toward them," says Galinsky. "And it gives us a really powerful tool to increase the intimacy. If this is someone that I know that I always hug this one way, and I wanted to be more connected to them, what if I hugged them a little bit differently the next time? Would that actually help our relationship?"

You'll never know until you try. Just...probably don't grab him by the genitals, baboon-style.

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