The “cuddle chemical”. The “moral molecule”. Oxytocin has quite a reputation – but much of what we thought about the so-called “love hormone” may be wrong.
Oxytocin is made by the hypothalamus and acts on the brain, playing a role in bonding, sex and pregnancy. But findings that a sniff of the hormone is enough to make people trust each other more are being called into question after a string of studies failed to replicate classic experiments.
Paul Zak at the Centre for Neuroeconomic Studies in Claremont, California, made his moral molecule hypothesis famous in 2011 when he memorably squirted a syringe of the hormone into the air while delivering a TED talk. When people sniff oxytocin before playing a money-lending game, it increases how much they trust each other, he explained.
But several teams have been unable to replicate his finding. Last November, Gideon Nave at the California Institute of Technology in Pasadena and his colleagues reviewed studies of oxytocin, and concluded that the effect of nasal squirts of the hormone on trust are not reliably different from zero.
Nave’s team aren’t the only ones calling the moral molecule hypothesis into question. In 2012, Moïra Mikolajczak at the Catholic University of Louvain (UCL) in Belgium and her colleagues published their own seminal findings backing a link between trust and oxytocin. They found that when people filled out an anonymous questionnaire about their sex lives and fantasies, they were less likely to seal the envelopes they returned them in if given a nasal dose of oxytocin beforehand.
It was a massive effect: those who received oxytocin were 44 times more likely to leave their envelope unsealed, suggesting that they trusted the recipient not to take a sneaky peak.
But now Mikolajczak’s team are casting doubt on their own findings, after they twice failed to replicate the results. This could be because their recent studies made it harder for individual participants to tell whether they were receiving oxytocin or a placebo.
But although the team improved their method and showed that their original finding might not be sound, the journal where their first study appeared decided it did not want to publish the details of their failed attempts to replicate it.
The team are now asking if there is a publication bias: are studies on squirting oxytocin up people’s noses more likely to be published if the result is positive? Their experiences suggest that the answer to that is yes. The team have revealed that of their 25 experiments on oxytocin, only the original questionnaire study suggested that intranasal oxytocin does affect trust.
These 25 studies yielded five published papers, only one of which reports a null finding – even though 24 out of their 25 experiments produced null results. That shows that the team has found it much harder to publish reports that squirting oxytocin has no effect. They have repeatedly sent a range of journals drafts of papers showing a null effect, but to no avail.
“Our initial enthusiasm for the [intranasal oxytocin] findings has slowly faded away over the years and the studies have turned us from ‘believers’ into ‘skeptics’,” the researchers write.
Nave suspects that it all comes down to probability, and has suggested that experiments like these are statistically equivalent to rolling a 20-sided die. Every time someone tests whether oxytocin works under certain conditions, they have a one in 20 chance of a positive result.
“If enough studies are carried out, every hypothesis will eventually be supported by some reports of experimental ‘evidence’,” Nave writes. When enough statistical tests are conducted independently, it is practically guaranteed that at some point, a desired result will appear.
If other people have had the same experience as Mikolajczak, then thousands of negative oxytocin findings could be hidden away in desk drawers. Other researchers support this view.
There are now questions over whether it is even possible for nasally delivered oxytocin to cross the blood-brain barrier. If not, then it’s unlikely that a squirt can have any powerful effect on behaviour.