Oxytocin Increases Trust in Humans: The Landmark Nasal Spray Experiment

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.

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.

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.

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.