Have you ever touched a loved one's arm and marveled at the softness of his or her skin? Turns out it's probably all in your head, a new study shows.
Researchers from the University College London found that people rated others' skin as softer than theirs, regardless of whether it actually was.
The findings, published in the journal Current Biology, reveal a sort of illusion that helps people form social bonds — by making the act of offering this simple tactile pleasure a reward in itself.
"Social touch plays a powerful role in human life, with important physical and mental health benefits in development and adulthood," the study authors wrote. "Touch is central in building the foundations of social interaction, attachment, and cognition, and early, social touch has unique, beneficial neurophysiological and epigenetic effects."
If you've ever shaken hands or hugged a friend, then you, too, have used touch as a social glue to connect with others. And scientists have long studied the many physical and psychological benefits of being touched: Physical contact with babies, for example, is key to their development. And yet, important as it is to feel touch, relatively little is known about what drives us to offer it.
"What does it feel like to give touch instead of receive it?" said senior author Aikaterini Fotopoulou, a cognitive neuroscientist at University College London. After all, she pointed out, "somebody needs to be motivated to do it."
Are there benefits to touching, not just to being touched?
To find out, the researchers set up a series of six experiments involving a total of 133 participants that would examine what people felt when they touched another's skin. They had them touch their own forearm and touch another person's forearm, and rate their softness. Sure enough, the participants regularly rated each other's skin as softer and smoother than their own.
When they tested their skin against the texture of other objects, they still rated the other person's skin as softer than theirs in relation to those objects.
They even attached scraps of cotton fabric to each participant's forearm and had the other stroke that, to make sure that the illusion was based on skin-to-skin contact. As expected, the participants rated both cloths to be equally soft and smooth.
The researchers also tested different speeds and found that the illusion seemed to work best with a slow, gentle caress, the kind you'd use to touch a baby or another loved one.
"Under these conditions, frequently encountered in intimate relations between partners, or between infants and caregivers, a social softness illusion arises: People have the persistent illusion that others' skin is softer and smoother than theirs, irrespective of any individual skin differences," the authors wrote.
Touching the other person's skin — and in particular, skin with hair follicles, not the bare skin like that of your palm — appears to be key.
This was surprising, Fotopoulou said. The person receiving touch may feel more pleasure from being touched on the forearm rather than the palm, because skin with hair follicles is full of C tactile afferents, nerves that pick up this kind of gentle movement.
But why would the people giving touch feel more pleasure? After all, they would be touching the other person using their fingers, where the skin lacks C tactile afferents.
It may be that the person who is touching the forearm has taken personal experience (i.e., I remember liking it when touched on the forearm, where there are lots of nerves to pick up the sensation) and projected it onto the person whose forearm they're touching (this must feel nice to you).
Humans, it seems, are programmed subconsciously to enjoy offering that kind of pleasurable contact, because they then perceive the other person's skin as more enjoyable to touch -- softer and smoother.
"Do things start in the skin or do they start in the brain?" Fotopoulou said. "This illusion seems to start in the brain."
This kind of caress is often pleasurable for the receiver. But this tactile illusion of softness ensures that the experience is enjoyable for the giver as well, the authors said.
"To the best of our knowledge, this novel illusion leads to the first demonstration in humans of the hedonic benefits of social touch for the touch provider," the authors wrote.
It allows the experience of touch to be a positive one for both people, enabling what the authors called a "tactile mechanism of emotional sharing."
We aren't the only species to need this kind of touch, the scientists pointed out. There's a reason that many primates sit in a line, grooming each other for long stretches of time.
"While remarkably little is known about what motivates pro-social touch in humans, in other mammals the motivational and functional aspects of a similar, active behavior — namely allogrooming — have been long investigated," they wrote.
"Non-human primates are known to spend far more time grooming their conspecifics than they actually need to for hygiene reasons, suggesting that allogrooming and its known beneficial effects on endogenous opioid release, pain, and stress alleviation, may have a role in promoting social bonds that in turn are important for survival."
Fotopoulou said she's interested in examining whether this experience diverges for different kinds of pairings -- be they two romantic partners, friends or strangers -- and examine the role it plays between mothers and their babies.