Have you ever caressed somebody's skin and wondered at how much softer it was than your own? If so, your senses may have been deceiving you.

According to research by University College London in the UK, our perception that others people's skin is softer than our own might be a sensory illusion, perhaps to ensure that humans are effectively motivated to touch one another and build social bonds.

In a series of experiments, the researchers placed participants in pairs and asked them to perform gentle stroking movements on each other's skin and to compare the softness, smoothness, and comfort of their own skin with that of their partner.

The participants consistently rated the skin of the other person as softer than their own, regardless of whether this was actually true.

"What is intriguing about the illusion is its specificity," said Antje Gentsch, co-author, in a statement. "We found the illusion to be strongest when the stroking was applied intentionally and according to the optimal properties of the specialised system in the skin for receiving affective touch."

The findings, published in Current Biology, suggest that the "social softness illusion" (SSI), as the researchers call it, takes place in the mind of the touch-giver as a kind of pleasurable incentive to reward touching.

"The illusion reveals a largely automatic and unconscious mechanism by which 'giving pleasure is receiving pleasure' in the touch domain," said lead author of the research, Aikaterini Fotopoulou.

The pleasure of touching one another may be the human equivalent of the excessive social grooming that takes place between family and group members in the animal kingdom (for example, primates grooming one another, birds grooming one another, and so on).

"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 behaviour — namely allogrooming — have been long investigated," the researchers write.

"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."