A very small new Finnish study attempts to shed light on the mechanisms of human bonding by taking brain scans of men in their underpants being stroked by their female partners, but not in sexually arousing areas.

Findings showed that this type of gentle contact activates chemicals in the brain's opioid system that may be critical for social bonding and can help the brain distinguish between strangers and the people we love.

"We know this is hugely important for humans because we have these strong, lasting bondings with friends and relatives and so on. But what kind of system maintains these bonds, and makes them last?" researcher Lauri Nummenmaa from Aalto University told The Guardian.

In their study, Nummenmaa and her team scanned the brains of nine men using PET scans, which can detect radiation emitted by a tracer molecule to contrast a 3D image of the brain, LiveScience reports.

In one test, the men lay in the scanner just in their underpants, while their female partners touched them gently all over, except in sexually arousing areas. In the second test, men lay alone in the scanner, with no sensual stimulation.

When the researchers compared the men's scans, they noticed that gentle stroking caused a drop in natural opioids in brain areas associated with the brain's reward circuitry. While the researchers expected the levels to spike, they said that opioids might work in a similar way to a painkiller, with the body needing less the more comfortable it was.

"The opioid system is typically engaged during pain, so you get a boost in painful situations," researcher Lauri Nummenmaa told The Guardian. "The social touching might be doing exactly the opposite. You can think of it as pain alleviation. That might be the underlying mechanism for why hooking up with others makes us feel so good in the first place."

The research team demonstrated their findings last week at the annual meeting of the Society for Neuroscience in San Diego, California.