New research suggests the 'love hormone' oxytocin may determine how sociable we are
Oxytocin junkies: The hormone that helps us bond with partners may also make social occasions enjoyable.
Can a single chemical be responsible for all the intimate connections we feel with other people? Oxytocin isn't called the "love hormone" for nothing. It has plenty of other functions, of course, among them triggering milk secretion during breastfeeding, and helping the cervix to dilate during labour. But it's oxytocin's role in bonding that is most intriguing.
Blocking the activity of oxytocin in rats or sheep causes mothers to reject their young. Injecting virgin female rats with oxytocin causes them to adopt other rats' pups as their own.
Oxytocin is also thought to be important in the formation of pair bonds. In humans its release can be stimulated by sex or simply by physical contact with a loved one, and it has a powerful relaxing effect. In prairie voles, blocking the effects of oxytocin prevents them from forming monogamous couples.
One study has even suggested that oxytocin might be involved in the bond between man and his best friend. Apparently, dog owners who are more attached to their pets have higher concentrations of oxytocin in their urine after interacting with them.
Other experiments have suggested that oxytocin could underlie trust. Subjects who inhaled oxytocin in a nasal spray were more likely to hand over cash to strangers, knowing that they might not get it back.
New research published in the journal Science this week has revealed more about oxytocin's role in social bonding. Finches, the subjects of this study, don't have oxytocin, but like other birds they have a very similar molecule called mesotocin.
When the scientists at Indiana University in the US gave drugs that block mesotocin receptors to zebra finches – which are normally highly social creatures – the birds spent much less time with familiar individuals and more time with unfamiliar individuals. They also preferred to hang out in smaller groups.
By contrast, zebra finches given extra mesotocin became more social and spent more time with familiar faces.
This fits with the idea of oxytocin acting to promote bonding between individuals. Intriguingly, the same paper suggests that the distribution of oxytocin receptors in the brain might help to explain why some animals are more social than others.
When the researchers compared three flocking finch species with two territorial, aggressive species, they found that the more social species had more mesotocin receptors in a part of the brain called the lateral septum. Blocking these receptors made the birds become less social.
Lead author James Goodson speculated that if the finding can be applied to mammals, then the concentration of receptors for oxytocin in the lateral septum might predict how gregarious somebody is likely to be.
"The lateral septum is structurally very similar in reptiles, birds and mammals," he said. "To our knowledge, it plays an important role in the social and reproductive behaviours in all land vertebrates."
Some scientists are interested in whether oxytocin can be used to improve relationships. Earlier this year, researchers at the University of Zurich found that couples given oxytocin before a "conflict discussion" displayed more "positive communication behaviours", such as eye contact and open body language, than couples given a placebo. They also had lower levels of the stress hormone cortisol.
Its involvement in social behaviours has even led to oxytocin being investigated as a potential treatment for autism.
But another recent study serves as a warning not to get too carried away. Researchers at the University of Haifa in Israel found that volunteers who inhaled oxytocin before playing a competitive game felt more envy when they lost and more schadenfreude when they won.
So while it's tempting to romanticise oxytocin, we would do well to remember that the "love hormone" isn't always a fomenter of happy relationships.