Oxytocin is often referred to as the "trust hormone," a claim that was reinforced by a 2005 study in which participants became more trusting after it was administered via a nasal spray. Trouble is, few studies have been able to reproduce this result, prompting at least one neuroscientist to suggest it's high-time we stop believing the hype.
The study in question, "Oxytocin increases trust in humans," was conducted by Michael Kosfeld and published in Nature. He wrote: "Here we show that intranasal administration of oxytocin, a neuropeptide that plays a key role in social attachment and affiliation in non-human mammals, causes a substantial increase in trust among humans, thereby greatly increasing the benefits from social interactions."
As part of the experiment, Kosfeld had his participants play the Trust Game — a standard two-player game in which one player takes on the role of an Investor (who is administered the oxytocin [OT]) and the other takes on the role of the Trustee. This experiment produced data showing a correlation — albeit a slight one — between OT and trust, or more accurately, the willingness of an investor to conduct transfers.
But as neuroscientist Mike McCullough points out in the inaugural post of his new blog, Social Science Evolving, follow-up studies have largely failed to reproduce these results. McCullough directs the Evolution & Human Behavior Lab at the University of Miami and is author of Beyond Revenge: The Evolution of Forgiveness.
Specifically, McCullough looked at five post-Kosfeld experiments (you can read his breakdown and analysis of them here). He came up with a scoring system and found that the original Kosfeld results have been succeeded by 1.25 studies' worth of successful replications and 3.75 studies' worth of failures to replicate. He concluded thusly,
With the relevant post-Kosfeld data favoring failures to replicate by 3:1, I think a dispassionate reader is justified in not believing that OT increases trusting behavior–at least not in the context of the trust game. Should we do a few more studies just to make sure? Fine by me, but it seems to me that we, as a field, should have some sort of stop-rule that would tell us when to turn away from this hypothesis entirely–as well, of course, as how much data in support of the hypothesis we would need to justify our acceptance of it. In addition, I'm struck by the fact that no one has ever gotten around to reporting the results of an exact replication of Kosfeld. In light of the Many Labs Projects' recent successes in identifying experimental results that do and do not replicate, I'd personally be content to believe the results of several (five, perhaps?) large-N, coordinated, pre-registered exact replications of the Kosfeld experiment. But until then, or until new data come in that are relevant to this question, I know what I am going to believe.
It's worth noting that McCullough is not debunking oxytocin as a trust hormone per se. All he's saying is that it appears unlikely that you can become more trusting by snorting OT through your nose.