Oxytocin Increases Trust in Humans: The Landmark Nasal Spray Experiment

What the researchers found

Intranasal oxytocin caused a substantial increase in trust behavior during a trust game, where participants had to decide how much money to entrust to an anonymous partner who could either reciprocate or keep it all.

Critically, the study demonstrated that oxytocin's effect was specific to social trust — it increased willingness to accept risks arising from interpersonal interactions, but did not increase general risk tolerance. This distinction suggests oxytocin targets the neural circuits involved in social approach behavior rather than simply reducing fear or caution across the board.

Why it matters

This study was a landmark in social neuroscience because it provided the first direct experimental evidence that a specific neuropeptide — oxytocin — causally increases trust in humans. It bridged decades of animal research on oxytocin's role in social bonding to human social behavior, and launched an entire field of research into the biological basis of trust, cooperation, and social decision-making.

How the study worked

Participants (adult males) received either intranasal oxytocin or placebo in a double-blind design, then played an experimental trust game involving real monetary stakes. In the trust game, one player (the 'investor') decides how much money to send to an anonymous partner (the 'trustee'), knowing the amount will be multiplied but the trustee can choose to return any amount — or nothing. Control conditions tested whether oxytocin affected general risk-taking independent of social interaction.

What this study cannot tell us

The abstract does not report specific sample sizes or effect sizes, making it difficult to assess the magnitude of the trust increase. The study only included adult males, so the findings may not generalize to women or other populations. The trust game, while validated, is an artificial laboratory setting that may not fully capture real-world trust dynamics. Later replication attempts in the oxytocin field have had mixed results, and the broader 'oxytocin and trust' literature has become more nuanced since this initial publication.

How to read the evidence

This is a randomized, double-blind, placebo-controlled experimental study published in Nature, one of the world's most prestigious journals. The experimental design with control conditions is rigorous, though the abstract lacks specific effect sizes and sample numbers.

When this study was published

Published in 2005, this is now over 20 years old. While it remains a foundational and highly cited study, subsequent research has significantly nuanced the 'oxytocin increases trust' narrative — oxytocin's effects appear highly context-dependent and may not be as universal as this initial study suggested.

The bigger picture

This 2005 Nature paper became one of the most cited studies in social neuroscience and helped cement oxytocin's popular reputation as the 'trust hormone' or 'love hormone.' It catalyzed hundreds of subsequent studies examining oxytocin's role in autism, social anxiety, PTSD, and bonding disorders. While later research has added nuance — oxytocin's effects depend on context, personality, and social cues — this study remains the foundational demonstration that a peptide hormone can directly modulate human social behavior.

Questions still open

• Does oxytocin increase trust equally in women, or are there sex-based differences in the peptide's social effects?
• Could intranasal oxytocin be therapeutically useful for conditions characterized by low social trust, such as social anxiety disorder or autism?
• What neural pathways mediate oxytocin's selective effect on social risk versus general risk?

Common questions

Read the original research

Oxytocin increases trust in humans.

Nature, 435(7042), 673-6

Citation

Kosfeld, Michael; Heinrichs, Markus; Zak, Paul J; Fischbacher, Urs; Fehr, Ernst. (2005). Oxytocin increases trust in humans.. Nature, 435(7042), 673-6.