Where to touch to make a person happy, scientists can now tell you. A team, including researchers from the Unilever company, have identified a class of nerve fibres in the skin that send pleasure messages.
And people had to be stroked at a certain speed - 4-5cm per second - to activate the pleasure sensation.
In the study published in Nature Neuroscience, they say the findings could help understand how touch sustains human relationships.
This research, which also involved experts at the University of Gothenburg in Sweden and at the University of North Carolina, recorded nerve responses in 20 people.
They then tested how people responded to having their forearm skin stroked at a range of different speeds.
They identified "C-tactile" nerve fibres as those stimulated when people said a touch had been pleasant.
If the stroke was faster or slower than the optimum speed, the touch was not pleasurable and the nerve fibres were not activated.
The scientists also discovered that the C-tactile nerve fibres are only present on hairy skin, and are not found on the hand.
Professor Francis McGlone, now based at Unilever after an academic career where he carried out research into nerve response, says this is likely to be a deliberate "design."
"We believe this could be Mother Nature's way of ensuring that mixed messages are not sent to the brain when it is in use as a functional tool."
He said the speed at which people found arm-stroking pleasurable was the same as that which a mother uses to comfort a baby, or couples use to show affection.
Professor McGlone said it was part of the evolutionary mechanism that sustained relationships between adults, or with children.
"Our primary impulse as humans is procreation, but there are some mechanisms in place that are associated with behaviour and reward which are there to ensure relationships continue."