Few molecules have a reputation as glowing as that of oxytocin. Often billed as the "love hormone" or "cuddle hormone", oxytocin has been linked to virtually every positive aspect of human behaviour. But it also promotes racial and cultural bias.

Despite its misleading labels, oxytocin has a dark side. Just two months ago, Jennifer Bartz showed that it can make people remember their mothers as less caring and more distant if they themselves are anxious about social relationships. Carolyn H. Declerck found that oxytocin makes people more cooperative in a social game, if they had met their partner beforehand. If they played with an anonymous partner who they knew nothing about, oxytocin actually made them less cooperative. "Oxytocin does not unconditionally support trust," she says.

Now, Carsten de Dreu from the University of Amsterdam has found that sniffs of oxytocin make us more biased towards peers from our own ethnic or cultural group, versus those from other groups. Bartz commends the new study, saying, "Along with other recent reports, [the new study] suggests that although oxytocin clearly plays a role in prosociality and empathy, the way it does this is more nuanced than previously thought. This is not entirely surprising given the complexity of human relations."



De Dreu asked 280 Dutch men to take three puffs form an oxytocin nose-spray, or a placebo that contained the same mixture without the hormone. It was a "double-blind" study – neither de Dreu nor the men knew who had been given what until the results were in.

First, de Dreu looked for any hidden biases in the volunteers' reactions to German, Arab or other Dutch men. He used an ‘implicit association test, where volunteers used two keys to categorise words into different groups (e.g. Dutch names or German/Arab names, or positive and negative). Combinations of categories that contradict our biases should subtly slow our reaction times. If people are biased against Arab people, they'd take longer to finish the test if the same key was assigned to both Arab names and positive words. These "implicit associations" are very hard to fake, especially if the test is done at speed.


Sure enough, oxytocin strengthened the biases of the Dutch volunteers. When they sniffed oxytocin (rather than the placebo), they were quicker to associate positive words with Dutch names than with either German or Arab ones.

Such biases can affect how we see other people. We humanise those who are part of the same group, ascribing a more complex range of emotions to them. By contrast, we have a tendency to dehumanise outsiders, by assuming that their emotional lives are narrower. This particularly applies to so-called "secondary emotions", such as admiration, hope or embarrassment, which are seen as unique to humans (in contrast to "primary emotions" like happiness, fear or disgust that are common to other animals).

De Dreu found that oxytocin strengthens these tendencies. He asked 66 white Dutch men to sniff either oxytocin or placebo before showing them pictures of other Dutch or Middle Eastern people. The volunteers had to say how strongly the people in the images would experience different emotions. Both groups were more likely to ascribe secondary emotions to people within their group than those outside it, but that difference was even greater after a sniff of oxytocin.

Finally, de Dreu showed that these shifting biases could affect the moral choices we make. He presented volunteers with a famous series of moral dilemmas. For example, a runaway rail trolley is hurtling towards five people who are about to be killed unless you flip a switch that diverts the trolley into the path of just one person. All of the dilemmas took the same form – you weigh the lives of one person against a group. And in all the cases, the lone person had either a Dutch, German or Arab name, while the group were nameless.


After a sniff of placebo, the Dutch volunteers were just as likely to sacrifice the single person, no matter what name they had. But after sniffing oxytocin, they were far less likely to sacrifice the Dutch loners than the German and Arab ones.

This last experiment clearly shows a trend that applied to the whole study: oxytocin boosted favouritism for people who belong to the same group. Only very rarely did it increase negative feelings towards people outside it. For example, in the moral dilemmas, oxytocin made the volunteers less likely to sacrifice members of their own group, but not more likely to sacrifice outsiders.



The stereotypical view of oxytocin is that it increases positive feelings rather than negative ones. But it doesn't do so equally. Rather than being an all-purpose chemical of social affection, you could view oxytocin as a drug that strengthens our tendency to discriminate between people within and outside our social cliques. "Different" can come under many guises – in de Dreu's experiments, Germans and Arab people were both treated as outsiders, despite having very different characteristics and stereotypes.

Bartz notes that oxytocin was simply amplifying the biases that volunteers already had. They already had a degree of favouritism towards their peers and oxytocin enhanced that. To Bartz, this suggests that the hormone could be heightening the importance of social cues in a person's environment, something altogether subtler than simply making everyone cuddly.


De Dreu's results fit well with previous studies, including animals ones. Under the influence of oxytocin, rat mothers become more aggressive towards intruders. Genetically engineered mice that can't respond to oxytocin develop a sort of "social amnesia", where they can't tell the difference between mice from their own group, and outsiders. Last year, De Dreu showed that oxytocin drives a "tend and defend" response. where people are more trusting and cooperative towards others within their group, but become more aggressively defensive to people outside it.

This sort of favouritism makes a degree of evolutionary sense. It could bolster trust and cooperation within a community, such that groups whose members stuck together more would out-compete those that did not. If that's the case, you would expect the brain to have some way of sustaining racial biases and oxytocin could help with that.


But such preferential treatment has an obvious dark side – it leads to all sorts of moral and cultural problems, including inequality, discrimination, prejudice, and conflicts between different groups. As de Dreu writes, "Oxytocin may trigger a chain reaction toward intense between-group conflict. This possibility questions the rather widespread view of oxytocin as a ‘cuddle chemical' or ‘love drug'." If it's a "love hormone", then that love is a narrow one.

Reference: PNAS http://dx.doi.org/10.1073/pnas.1…

Dive into the awe-inspiring, beautiful, and quirky world of science news at Discover's Not Exactly
Rocket Science blog, written by Ed Yong. No previous experience required.