Hugs, handshakes, butt pats: Touching among humans is, well, a touchy subject. As kids we were taught that there’s good and bad touches—and it only got more complicated from there. Nobody is immune to the vagaries and complications, either: How politicians touch—from former President George W. Bush's famous Merkel-shoulder-rub gaffe to Michelle Obama's half-embrace of the Queen—is often controversial, and there's seemingly never-ending debate in business circles about hugging vs. handshaking (and let's not even get into single- vs. double-cheek kissing). At the same time, touching is essential to human bonding, stimulating oxytocin and endorphins.

Perhaps science can sort this out? Here are some rules on the subject from research.

RULE #1: TOUCH ISN'T THE SIMPLE SENSE WE THINK IT IS.

For a long time, it was believed that touch was the “simple sense”: that the brain, and specifically the somatosensory cortex, interpreted basic information like temperature and pressure as received by the skin—and that’s it. One 2012 study of heterosexual males showed that the brain processes touch using more cues than just physical sensation. In the study, the subjects received a "sensual caress" from an unseen hand while watching a video clip of either a woman or a man who appeared to be delivering the well-timed touch. In reality, every stroke was delivered by a woman's hand. Looking at fMRIs of the men's brain activity, researchers saw that their somatosensory cortex responded more significantly to what they believed was the woman’s touch than to the man’s. So that “simple” part of the brain wasn’t just interpreting physical cues, but was also accounting for cultural and emotional information. As far as the brain was concerned, those aspects of touch were inseparable from physical stimulation. 

RULE #2: FEELINGS MATTER MORE THAN FREQUENCY.

A study published recently in PNAS determined that it’s the closeness of the relationship, rather than frequency of seeing a person, that determined how acceptable intimate touch was—which surprised researchers, until they realized why. Lead author Robin Dunbar, of the Department of Experimental Psychology at Oxford, tells mental_floss that the results make sense when you consider that “what is important in a relationship is how you feel about the person. The frequency of contact is simply the vehicle to achieve that, not the thing itself,” he says.

RULE #3: TOUCH IS A LANGUAGE TOO.

In a 2006 study [PDF] that involved participants in the United States and Spain, random pairs of strangers were separated, with just a black curtain between them. One was given the task of communicating an emotion by touching the other person’s hand or arm. Researchers found that the people being touched “could decode anger, fear, disgust, love, gratitude, and sympathy via touch at much-better-than-chance levels." Another study by the same researchers found that people could accurately decode distinct emotions by merely watching others communicate by touching each other. In short, touch gave just as much information as tone of voice or facial expression.

RULE #4: WOMEN ARE MORE COMFORTABLE BEING TOUCHED AND TOUCHING.

Naturally, this is not permission for anyone to touch women more (I’m talking to you, creepy subway guy and inappropriate co-worker), but according to Dunbar’s PNAS study (see #2), women are—in general—both more comfortable being touched and more likely to touch others. This touchiness appears to cement deeper connections: "participants reported feeling stronger emotional bonds with female than male members of their social networks," according to the study.  

RULE #5: WE'RE MORE SIMILAR THAN DIFFERENT.

The same study also found that the rules of touch don’t vary as much as we think. While his study only looked at European countries, the expected differences between, for example, Finns and Italians—traditionally thought to be on either side of a great European touch divide, with northern cultures hands-off and Mediterranean cultures hands-on—was smaller than anticipated, suggesting the basics of touch are less of a cultural artifact than we might assume. 

RULE #6: EXPECT TO MAKE MISTAKES.

Touching others is just fundamentally confusing: “…we are always caught between two things—that we express closeness by more touch and that we try to use touch to express more closeness (when it doesn't exist but we would like it to). So it is always a bit risky, and that is why people get into trouble,” Dunbar tells mental_floss. His best advice for reaching out? “Be careful! Read the signals first!” Perhaps we should update the old school rule: Until you know what's acceptable, keep your hands to yourself.