Oxytocin – often referred to as the “love hormone” – also plays a role in competitions between groups, a new study led by Charlotte Debras and Adrian Jaeggi from UZH’s Institute of Evolutionary Medicine shows. “Previous work showed that receiving oxytocin via a nasal spray can make people more ‘groupish’,” says Debras, who conducted the study as part of her doctoral research. “But it wasn’t known whether OT naturally increased during group competition.”
Oxytocin levels linked to social context
To systematically study competition, the researchers organized football tournaments together with the Indigenous Tsimane’ people in the Bolivian Amazon. “The Tsimane’ lifestyle has an effect on their hormone levels,” says Jaeggi, who has been conducting research on site since 2011. For example, levels of reproductive hormones like testosterone or progesterone are lower when calories are harder to come by. Meanwhile, the kinds of social factors linked to OT are likely especially important in these highly interdependent societies. To test their hypothesis, the researchers measured oxytocin levels in the urine of football players before and after the matches.
Oxytocin levels increased most strongly after matches against familiar rivals but less so among teams from different communities. When Tsimane’ played against non-Tsimane’ people, however, the increase was again higher. “This suggests that oxytocin is sensitive to the salience of the opposition – showing heightened reactivity both toward familiar competitors and toward a clearly defined out-group,” says Jaeggi.
Higher oxytocin levels in men
One particularly striking finding of the study was that women’s OT levels did not differ before or after games, unlike men’s. Debras mentions several possible explanations for this. “Firstly, women start from higher baseline levels, as many of them were breastfeeding, which could make it harder to detect a change.” It is also possible that football may not hold the same significance for women, as they play less often than men. Another explanation is provided by the “male warrior hypothesis,” which proposes that men have evolved to be generally more invested in group competition. “The most important forms of rivalry for Tsimane’ women may involve relational dynamics, such as managing reputations or jostling for social support, rather than physical competition,” says Debras.
Relevant for cooperation and group boundaries
The study does not make it possible to determine whether the increase in OT primarily promotes cohesion within the team or competition with the opposing group. In team sports, both aspects are closely intertwined. “Cooperation can be a successful way to compete in the game of life, and OT seems to be a key ingredient for this,” says Jaeggi. “OT has been linked to group conflict in many different species, from fish to chimpanzees. Our findings suggest that similar mechanisms are at play in humans.”
Literature
Charlotte C. Debras et al. Us against Them: Oxytocin Response to Competition in a Small-Scale Human Society. Proceedings of the Royal Society B. 6 May, 2026. DOI: https://doi.org/10.17605/OSF.IO/UK8PA
Contact
Prof. Adrian V. Jaeggi
Institute of Evolutionary Medicine
University of Zurich
Tel. +41 44 635 05 40
E-mail: adrian.jaeggi@iem.uzh.ch