Oxytocin, often called the love hormone, makes men with low trust levels more willing to trust strangers.
Trust allows people to cooperate without knowing exactly what someone else will do. This uncertainty creates opportunities, but it also leaves room for betrayal.
Oxytocin, sometimes called the “love hormone,” is a natural chemical messenger that can influence social behavior and may also affect trust.
A new trial found that a nasal dose of oxytocin increased trusting behavior among men who scored relatively low on a measure of general trust.
The effect appeared in a controlled money game, so it does not mean that Hormone It makes everyone more confident in daily life.
Why were previous results inconsistent?
In 2005, a team led by Ernst Fehr V University of Zurich reported that oxytocin increased confidence in men playing the money game.
Subsequent experiments produced mixed results, with some supporting this finding and others detecting no widespread effect.
Many experiments included too few people to get clear answers. Some teams formed their main interpretations after seeing the results, while different approaches made direct comparisons difficult.
The disagreement has left a fundamental question unanswered: Does oxytocin increase trust, and does it only work under certain conditions?
Larger testing gives clearer evidence
The new team recorded its master plan for forecasting and analysis before looking at the results.
This process, called pre-scoring, reduces the chance that researchers will reformulate the question after knowing the data.
A six-question survey measured dispositional trust, which means a person’s habitual willingness to trust and the belief that others will respect that trust.
Only men who scored 38 or lower were invited into the lab, which allowed the researchers to focus on participants with relatively low personal confidence.
The final sample included 359 men, giving the study much greater statistical power than many previous trials.
Each man was randomly assigned to receive either 24 IU of synthetic oxytocin or a placebo nasal spray without the active ingredient.
Neither participants nor staff knew what type of spray each man received, which helped prevent expectations from shaping the results.
Money game measures trust
After waiting for the spray to activate, each participant entered an anonymous trust game.
He started with 12 euros and could send zero, four, eight or 12 euros to an unknown partner called the trustee.
The experimenters tripled the money sent before handing it over to the trustee, who could return any amount. Sending more created the opportunity for greater gains, but also increased the risk of losing money.
Since both players started with the same amount, the transfer measured willingness to risk someone else’s reaction rather than mere charity.
The players remained in isolated compartments and never knew each other’s identities. Faces, conversations, and other social cues were unable to shape the decision.
Other than oxytocin options
The men who received oxytocin sent an average of €8.29, about 15% more than the €7.20 sent by men in the placebo group.
The result remained when the team took into account personality traits, risk preferences, mood changes, and guesses about the type of spray each person received.
The examinations also showed that there was no single laboratory session that drove this pattern.
The researchers then combined the new data with 219 similar men from a previous, anonymous trial, creating a total sample of 578. Across that larger group, oxytocin increased average conversions by 16.9%.
“After more than 20 years, we have now succeeded in providing strong evidence of the selective effect of oxytocin,” said Ernst Fehr.
The effect of oxytocin was selective
Within the low confidence group, men with the lowest survey scores did not show a stronger response than men closest to the cutoff. Oxytocin appears to change behavior by a similar amount across the range.
The new experiment did not include men with moderate or high confidence scores, so it cannot show on its own that these groups would not respond.
However, a previous trial in 2020 found no overall effect and no increase among men who had higher confidence in the unknown environment.
Oxytocin also did not change how much the trustees returned after receiving the money. Its measured effect appeared in the initial decision to take a social risk, not in the subsequent repayment process.
Possible causes are still unclear
“It is possible that this hormone increases expectations of others’ cooperation,” Ernst Fehr said.
Oxytocin may change how strongly a person expects a fair return. It may instead reduce the fear of being cheated, even when the expected chance of repayment remains the same.
The hormone can also affect brain systems associated with reward, uncertainty, and fear, or body systems associated with calm social engagement. The experiment did not measure these processes directly.
The team did not scan participants’ brains or ask them to report their expected payment before choosing the amount. Therefore, these potential explanations remain untested.
There are still important limitations
The trial included only men because the team wanted to match previous oxytocin results. Therefore, it cannot be assumed that the results apply to women.
Participants came from university students, and the game involved anonymous decision-making without any prior communication.
Confidence in families, workplaces, or frequent Relationships It may follow different patterns when people are able to see and evaluate each other.
The trial tested a single dose over a short period. It did not study repeated use, permanent personality change, or whether oxytocin could be a safe and useful treatment.
Oxytocin may selectively increase confidence
In this context, random assignment supports a direct effect of oxytocin on the amount of money entrusted by men with low dispositional trust.
The hormone does not appear to improve and strengthen judgment Relationshipsor creates confidence in every situation.
The main lesson is that oxytocin does not seem to act as a universal trust switch. Its effects seem to depend on the person’s level of confidence and may also vary depending on the social environment.
Future work should include women, broader populations, and direct measures of expectations and fears. These steps can test whether the same selective pattern applies beyond an anonymous laboratory game.
The study is published in the journal Proceedings of the National Academy of Sciences.
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