A social bonding hormone boosts cooperation among men interacting with women

A recent experiment reveals that a hormone linked to social bonding causes heterosexual men to cooperate more frequently with female partners in a computerized game, regardless of whether the women are presented in a sexualized manner. The researchers discovered that the hormone particularly encourages men to forgive betrayals under specific conditions, shedding light on the biological underpinnings of social and mating behaviors. These results were published in Psychoneuroendocrinology.

Society is heavily saturated with sexualized imagery, prompting psychologists to investigate how emphasizing a person’s sexual nature alters social perception. Often, sexualization leads to sexual objectification. This cognitive shift occurs when an individual is reduced to the status of a mere object or body part, stripped of their humanity and capacity to direct their own actions.

Objectification generally leads to negative social outcomes, such as reduced empathy and a lower likelihood of collaboration. When sexualization does not cross the line into objectification, it can serve as a highly rewarding social stimulus. In these harmless scenarios, sexual attractiveness might elevate a person’s social value and draw increased attention from members of the opposite sex.

Gonçalo Cosme, a researcher at the University of Lisbon, and Diana Prata, a neuroscientist at the University of Lisbon and King’s College London, wanted to understand what tips the balance in these interactions. They designed a study to determine whether sexualization invites or deters cooperation from heterosexual men during an economic game. They also sought to test the specific influence of oxytocin on these behaviors.

Oxytocin is a chemical messenger produced in the brain that modulates social cognition, partner bonding, and trust. Because oxytocin levels naturally spike during physical intimacy, the research team suspected it might play a role in navigating sexualized social contexts. The hormone is associated with an increased desire to approach others and build cooperative relationships.

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To investigate these dynamics, the researchers recruited 50 heterosexual men for a double-blind, placebo-controlled experiment. Half of the participants self-administered a nasal spray containing synthetic oxytocin, while the other half received a placebo spray with no active ingredients. Neither the participants nor the researchers knew who received which treatment until the experiment concluded.

Each participant played a classic game theory exercise called the Prisoner’s Dilemma. The Prisoner’s Dilemma represents a situation where two individuals must independently weigh the benefits of mutual collaboration against the temptations of selfish betrayal. The participants played multiple rounds of this game against two supposed female partners located in another university room. The researchers required the participants to make the first move in every encounter, placing the men in a vulnerable position that required a baseline level of trust.

If both players cooperate in the game, they both earn a moderate hypothetical reward. If one betrays while the other cooperates, the betrayer receives the maximum reward while the victim receives nothing. If they both betray each other, they both receive a very small reward. This setup forces players to consider their partner’s likelihood of acting in good faith.

In reality, the men were playing against a computer algorithm designed to mimic human decision patterns. Before each round, the participants saw a photograph of their opponent. One algorithm was represented by a sexualized woman wearing heavy makeup, a short dress, and high heels. The other algorithm was represented by a non-sexualized woman wearing a simple sweater, comfortable trousers, and flat shoes.

While the men played the game, they wore a cap covered with electrodes to measure their brain waves. This recording technique, called electroencephalography, allows researchers to observe how the brain evaluates outcomes in fractions of a second. The researchers monitored the timing and size of electrical spikes to understand the biological mechanisms driving the men’s behavioral choices.

In this experiment, the team monitored two specific electrical events. First, they looked at the P300 brain wave, which is a sudden positive shift in electrical voltage that occurs roughly a third of a second after a person sees something unexpected or arousing. The second event was the Feedback Related Negativity wave. This involves a negative shift in voltage that appears when a person realizes they have experienced an unfavorable outcome.

To ensure their experimental setup worked properly, the researchers asked participants to fill out questionnaires assessing the presumed mental states of their opponents. Psychological metrics divide perceived humanity into the capacity to experience emotions and the capacity for personal agency. The latter category evaluates the ability to plan, make choices, and exert self-control.

When someone is completely objectified, observers tend to view them as having a highly diminished capacity for personal agency. In this study, the men rated the sexualized women as more attractive and sexually available. Because they did not score the sexualized women lower in personal agency, the results confirmed that the women were sexualized but not stripped of their autonomy.

The administration of oxytocin altered the men’s behavior entirely independently of the women’s appearance. The men who received the oxytocin spray chose to cooperate more often across the entire Prisoner’s Dilemma game than those who received the placebo. The hormone consistently boosted their baseline willingness to trust a stranger.

The hormone specifically increased the men’s willingness to maintain mutual cooperation after a round where both players had acted generously. It also elevated the likelihood of a participant forgiving an opponent who had just betrayed them. Choosing to cooperate immediately following a betrayal requires a high degree of tolerance. This forgiving behavior indicates an internal motivation to repair a damaged social bond.

This forgiving behavior heavily depended on the order in which the opponents appeared during the experiment. The oxytocin-fueled forgiveness occurred most often when the men played against the non-sexualized woman first, before encountering the sexualized woman. The team suspects that encountering a non-sexualized partner right after a sexualized one might feel like a lowered social reward for the participant. This perceived disappointment could reduce the internal motivation to forgive a betrayal.

The brain wave recordings provided physical evidence of these social evaluations. When the men saw the outcomes of rounds played against the sexualized women, their brains displayed higher amplitudes in the P300 electrical signals. This suggests that the men dedicated more cognitive attention to interpreting the actions of the sexualized partners.

When men under the influence of oxytocin played against the non-sexualized women, their P300 brain signals showed a delayed response time. A longer delay in this specific brain wave pattern typically indicates that the brain is taking more time to evaluate feedback. This delay hints that processing an ordinary social interaction requires a higher cognitive load when the participant is biologically primed for social bonding.

The researchers observed that the Feedback Related Negativity brain waves were much stronger when the men evaluated losses in the game. These negative voltage shifts were also exceptionally high when the men played against the non-sexualized women as their final opponents. The researchers propose that this spike reflects a subconscious negative evaluation of the non-sexualized woman following a prior interaction with a sexualized one.

The study provides a biological perspective on social trust, but the authors acknowledge a few limitations in their work. The experiment only tested the cooperative behaviors of heterosexual men toward female partners in a controlled setting. The dynamics of sexualization and oxytocin might operate differently for women or for individuals with other sexual orientations.

The participants all shared a similar demographic background, largely consisting of young university students in Portugal. Cultural backgrounds shape how people interpret clothing and sexual availability, meaning these results might not perfectly replicate in other societies. The researchers also used a single standard dose of oxytocin, leaving room for future experiments to test whether differing amounts of the hormone produce varying effects.

Future investigations could expand on this framework by observing real-life social interactions rather than computerized economic games. The current statistical findings from the brain wave data are considered preliminary and require independent replication. Until then, these findings offer an updated perspective on how our neurobiology navigates the boundaries between trust, attraction, and social cooperation.

The study, “Intranasal oxytocin increases cooperation of heterosexual men with women,” was authored by Gonçalo Cosme, Marta Patrocínio, Carlotta Cogoni, Maciej Kosilo, and Diana Prata.