Is Oxytocin the “Trust Molecule"?

The effects of oxytocin on behavior may not be so simple.

Posted Dec 14, 2015

Trust (466709245)" by Terry Johnston from Grand Rapids, USA - Trust. Licensed under CC BY 2.0 via Wikimedia Commons
Source: Trust (466709245)" by Terry Johnston from Grand Rapids, USA - Trust. Licensed under CC BY 2.0 via Wikimedia Commons

People love simple stories about human psychology.  They get touted in the news all the time.  Play brain games and get smarter.  Meditate and watch your stress melt away. 

One simple story that has gotten a lot of attention in the popular press surrounds the hormone oxytocin.  Oxytocin is a hormone that plays many roles in the human body.  For example, in large doses, it causes contractions of the uterus during childbirth and plays a role in the letdown of milk in nursing mothers. 

Researchers have also been interested in behavioral effects of oxytocin.  In animals, oxytocin has been demonstrated to play a role in pair bonding of animals. Based on these findings, a number of studies have explored whether oxytocin increases trust in humans.  Based on some preliminary evidence, Paul Zak actually dubbed oxytocin “The Trust Molecule.” 

This question was explored in detail in a review of research by Gideon Nave, Colin Camerer, and Michael McCullogh, published in the November, 2015 issue of Perspectives on Psychological Science

They reviewed three kinds of research that explore the link between oxytocin and trust.  One set of studies administers oxytocin by having participants breathe it in through the nose using a mister.  A second set of studies measured oxytocin levels in the blood and related those to people’s overall level of trust.  A third set of studies looked at genes associated with oxytocin levels and correlated those with measures of trust.

The excitement about this area of research came from an initial study in which participants were part of a 2-player game from behavioral economics called the trust game. In the trust game, the experimenter gives some money to Player 1 (say $10).  Player 1 can give as much of that money to Player 2.  The experimenter then multiples the amount give by 3, and gives it to Player 2.  Player 2 can then return any amount of money to Player 1.

The reason this game is called the trust game is that the best outcome for both players is if Player 1 gives the entire $10 to Player 2.  That $10 because $30 when the experimenter multiples it by 3.  If Player 2 then splits the money, each player ends up with $15.  But, in order or this to work, Player 1 has to believe that Player 2 will return at least as much Player 1 invested. 

An early study compared people who inhaled oxytocin to those who inhaled a placebo.  When the person playing Player 1 inhaled oxytocin they invested more money than when the person playing Player 1 inhaled a placebo.  Oxytocin did not affect how much money Player 2 returned to Player 1.

This study seems like good evidence that Oxytocin is affecting trust.  Unfortunately, several subsequent studies using similar methods failed to find any effect of inhaled oxytocin on performance in the trust game.  In addition, studies that looked for effects of inhaled oxytocin on questionnaires looking at how trusting people are did not find an effect of oxytocin on trust.

The authors also reviewed several studies looking at levels of oxytocin in the bloodstream.  Unfortunately, it turns out that the all of these studies used a flawed method for evaluating the levels of oxytocin in the bloodstream.  Nonetheless, there was no strong association between oxytocin levels observed in these studies and measures of trust.

Finally, some studies have looked at a genetic variation in a gene that affects oxytocin receptors in the brain.  The idea was that these genetic variations might lead to trait differences in trust levels.  An early study did report a correlation between a particular gene allele and Player 1 investment in the trust game.  Other small studies found relationships between genes related to oxytocin receptors and other social behaviors.  However, a large-scale genetic study failed to replicate those effects.

When you put this all together, it appears that there is no good reason to think that oxytocin is directly related to trust.  That does not mean that oxytocin has no influence on human behavior.  It is just that the actual relationship between oxytocin and behavior is likely to be complicated.  It will probably interact with situations and other aspects of people’s brain structure. 

More generally, it is probably worth being skeptical of any simple relationship between a chemical or a manipulation and a behavior.  Human behavior is just more complicated than that.

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