But recently, several scientists have shown that this tower of evidence for oxytocin's positive influence is built on weak foundations.

Gideon Nave at the California Institute of Technology found five other papers where researchers had used similar trust games to those in the original Nature experiment. None of these found that a sniff of oxytocin could significantly boost trust. And when the team combined the results of all six studies, they couldn't find an effect either. “We were interested in whether the foundational finding that got this research focus going was firm,” says Michael McCullough at the University of Miami, who was involved in the work. “And the odds that it's true are rather low.”

Ernst Fehr from the University of Zurich, who led the original Nature study, notes that none of the other studies cleanly replicated his own. Each introduced tweaks, and sometimes flaws, that would have changed the results. Still, he says, “What we're left with is a lack of evidence. I agree that we have no robust replications of our original study, and until then, we have to be cautious about the claim that oxytocin causes trust.”

Nave's team also criticize the methods used in the tidal wave of human oxytocin studies. For the hormone to exert an influence on behavior, it presumably needs to reach neurons in the brain, and dock at proteins called oxytocin receptors. No one knows if the nasal sprays that have been the bedrock of human studies can actually deliver oxytocin to those neurons. The hormone certainly enters the body in large amounts, but it barely crosses the barrier that separates our blood supply from our brain—a view supported by another recent review.

These criticisms don't just apply to studies on trust, but to those on altruism, cooperation, and other behaviors that oxytocin supposedly boosts. When Larry Young from Emory University analyzed a wealth of past studies using oxytocin nasal sprays, he found that they are very statistically underpowered. Statistical power refers to the odds that a study will find an effect, provided one exists. Ideally, scientists aim for a statistical power of at least 80 percent, giving a 4 in 5 chance of finding an effect. But Young showed that studies with oxytocin nasal sprays were so small that they had an average power of just 16 percent. So, the fact that most of them reported statistically significant effects becomes suspicious, given how unlikely they were to find any.

Indeed, Young has personally talked to people who tried oxytocin-sniffing experiments, failed to find anything, and never published their results—a common problem that plagues many areas of science. “The field's in its adolescence, where there's a growth spurt and not a lot of prefrontal control,” says Young. “The best decisions aren't always being made.”