An international team of researchers has found that the “love hormone” oxytocin has more nuanced effects on the brain than previously thought.

The team, including Yale professor of psychology, economics and management David Rand, gave participants a dose of oxytocin and found that the chemical’s effect on decision-making differed based on the way the participants thought about those decisions. They found that, after being exposed to oxytocin, people who were “intuitive” decision makers were more likely to favor members of their own group when cooperating, while those who were “reflective” decision makers tended to show less in-group favoritism in cooperating. The research will be published in the journal Neuropsychopharmacology.

“We were trying to show whether oxytocin affects social cognition,” said study lead author Yina Ma, a post-doctoral fellow in the Cognitive Neuroscience Division of the Lieber Institute for Brain Development. “Oxytocin is a key neuropeptide that plays a key role in social cognition but the literature has shown inconsistent results, so we are trying to explain why — one of the key things we are thinking about is cognitive style.”

Previous research had found that oxytocin and intuitive decision-making activate similar parts of the brain. The literature on the effects of oxytocin on in-group favoritism, however, had been mixed with some studies showing an increase in in-group favoritism and some showing no effect from the oxytocin.

But past research had definitively shown that intuitive decision makers — those who prefer fast heuristic decision-making methods — tend to cooperate with members of their own group in comparison to cooperating with members of an outgroup. Meanwhile, reflective thinkers — those who use slower analytic methods — show less in-group favoritism. Those facts, along with questions about why research on in-group favoritism has been inconclusive, allowed the researchers to hypothesize that the inconsistencies could be explained by different ratios of intuitive to reflective thinkers in sample sizes. The more intuitive thinkers there are, the greater the observed effect on in-group favoritism, and the more reflective thinkers there are, the less the observed effect.

To test that hypothesis, the researchers tested a group of 150 Chinese men in a double-blind study, giving them either an oxytocin nasal spray or a placebo. They found intuitive thinkers were more likely to increase in-group favoritism. Among reflective thinkers, the effect was reversed, verifying the researchers’ hypothesis.

Ma said these results will be instrumental in understanding how oxytocin affects the brain before researchers can begin to look into clinical applications. She said a future project may be to closely examine the anti-anxiety effects of oxytocin.

According to Kevin Pelphrey, director of the Yale Child Neuroscience Lab, who was not involved in the study, studies like this could be particularly helpful for treating autistic children. Studies measuring the effects of oxytocin on autistic children are currently being conducted, but if these treatments become standard, researchers need to understand that kids in different contexts will react differently to oxytocin, he said.

Currently, pharmaceutical companies are working on methods of increasing the oxytocin levels in the brain without having to directly dose a patient with oxytocin itself, allowing researchers to not have to depend on nasal sprays, Pelphrey said.

Rand said in an email that the subfield of oxytocin research should continue to become more interdisciplinary, forming stronger connections between the fields of biology and psychology.

He added that it would be interesting to conduct the same research on American participants to see whether the results differ.

Oxytocin is released in the body after sex and during maternal bonding.