Researchers have shed light on the
chemistry that bonds one person to
another by taking brain scans of men
being stroked while in their
The Finnish study found that
gentle stroking – which was not in
sexually arousing areas – changed
levels of opioid brain chemicals
which work behind the scenes to form
lasting bonds in animals.
The findings suggest that opioids
might be the critical chemicals that
enables human brains to distinguish
between strangers and people who
are closer to us, such as friends,
families and lovers.
"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?"
said Lauri Nummenmaa who studies
the neural circuitry of emotions at
Aalto University in Finland.
Studies in animals have shown
that opioids can play a crucial role in
pairing up. Prairie voles are
monogamous in the wild, but when
given a drug that blocks opioid in
their brains, they seek out other
partners. If opioids are blocked in
monkeys, they groom others less and
neglect their babies.
To see whether opioids were
important in human bonding, the
researchers invited nine couples into
the lab. The men stripped off to their
underpants and lay under a blanket
in a PET scanner. The first scan was
taken while the men were alone. For
the second, their partners touched
them gently all over, but avoided
anywhere likely to arouse them
"I'm really proud of the Finnish
general public," said Nummenmaa.
"We had no problem whatsoever in
recruiting people for the
When the researchers compared
the men's scans, they noticed that
gentle stroking caused a drop in
natural opioids in brain areas called
the ventral striatum and the anterior
cingulate cortex, which are mainstays
of the brain's reward circuitry. This
was counter to expectations: they had
expected levels to rise.
Nummenmaa said that opioid
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. 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," said
Nummenmaa. Details of the study
were given at the Society for
Neuroscience meeting in San Diego .
Opiate drugs have the same
effect as the body's natural opioids,
and just as people build up tolerances
to drugs, develop cravings, and suffer
withdrawal symptoms, similar
processes play out with relationships.
"If something similar happens
when you are establishing social
bonds that would make perfect sense.
You would need your daily fix, or at
least a fix now and then, and unless
you got that you would start to feel
pain," said Nummenmaa.