Oxytocin and human behavior – or how a hormone can (or can’t) influence the distance between man and woman upon a first encounter.

A couple of weeks ago, I came across a piece of news in a popular science magazine about the hormone oxytocin and how it affects the behavior of men in relationships towards unknown women. Not surprisingly, I subsequently saw that several media had picked up on the story. After all, questions of fidelity and maintenance of long-term relationships usually raise a certain amount of interest in people. The headlines ranged from a rather neutral report of the study’s main conclusion (oxytocin affects the distance men in relationships keep from unknown women) to slightly more far-fetched – and surely catchier – claims (oxytocin may help keep men from cheating).

Rather intrigued, I checked out the original study, which was published on November 14 in the Journal of Neuroscience.

Oxytocin is a small molecule synthesized in a region of the brain called the hypothalamus and is known to play an important role in triggering childbirth and facilitating nursing. Oxytocin is also known to be important in social interactions, promoting bonding. It has received the nickname of “love/trust hormone” after studies revealed that it promotes bonds between parents and children and between partners (in humans but also in other mammals), and that it increases trust among people.

Previous studies in humans have suggested a role for oxytocin in attraction and bonding behavior between men and women. Research in monogamous mammals other than humans, in particular the prairie vole, a rodent species, has shown that oxytocin promotes the formation of pair-bond in females (a related molecule called arginine-vasopressin seems to be more important in males). It has also been shown that the basal blood levels of oxytocin in humans are more elevated in couples compared to single people, both in the early stages of a relationship and 6 months later.

Considering that oxytocin seems implicated in the formation of pair-bonding, the authors of the study wondered whether oxytocin was also involved in the maintenance of monogamous relationships in humans. Further narrowing their study subject, they asked whether oxytocin would affect the behavior of men towards unknown women and whether the men’s relationship status would influence any effect oxytocin might have.

To address these questions, they set up an experiment where they measured the distance heterosexual men kept between themselves and an unknown woman after inhaling either a dose of oxytocin or a placebo. The experiment was done in four ways, with either the female experimenter moving towards, or away from, the study subject (the man), or the study subject moving towards or away from the female experimenter. In all 4 cases, the men were asked to stop or tell the female experimenter to stop when they felt the distance between themselves and the woman was comfortable.

The researchers found that men who were in relationships and who had inhaled oxytocin stayed at a greater distance from the female experimenter than men who were also involved with someone but had inhaled a placebo (10-15 cm further on an average distance of 55-60 cm). This effect of oxytocin was observed only in men who were in relationships, as single men who had inhaled oxytocin kept the same average distance between themselves and the female experimenter as single men who had inhaled a placebo. Also, oxytocin had no effect on the distance men in relationships kept from an unknown man, indicating that the effect of oxytocin observed in the first experiment with the unfamiliar woman was not a general one influencing all social interactions, but was perhaps an effect more specifically related to pair-bond (relationship) maintenance.

The men were later asked what they thought of the female experimenter, and all men rated her as highly “likeable” and “trustworthy”, regardless of whether they were single or not, and whether they had inhaled oxytocin or a placebo. This suggests that oxytocin did not keep men in relationships at a greater distance from the unknown woman by altering their conscious perception of this woman.

Altogether, the authors of the study draw two main conclusions: 1) the effect of oxytocin on human behavior is influenced by external factors, in this case relationship status, and 2) oxytocin may play a role in the maintenance of monogamous relationships in humans by selectively influencing men involved with someone to stay a bit further away from unfamiliar women.

Now, let’s recall that the basal levels of oxytocin are higher in men in relationships than in singles. However, in the study, men in relationships stayed at the same distance from the unknown woman as single men did if they had inhaled a placebo. This means that they still needed an extra shot of oxytocin just before encountering the unknown woman to stay at a greater distance from her. The most obvious physiological way of stimulating the release of oxytocin in these men would be for them to have sex with their partner. However this might not be the easiest or most appropriate thing to do, say, just before they make a new (female) acquaintance at the birthday party of their 5-year old nephew. Probably also taking into account that oxytocin nasal sprays are not quite the average over-the-counter drug, the authors of the study suggest that just the close presence and touch of their partner might be enough. I wonder whether there is any research study out there showing that holding your partner’s hand induces a detectable increase of oxytocin in his/her blood …

On the whole, oxytocin is probably not the wonder molecule that will prevent men from cheating, as some article headlines would make you believe. But the study leads to two interesting observations: although oxytocin promotes bonding and social interactions, 1) it did not bring men and women closer (in a spatial sense) upon a first encounter, regardless of the men’s relationship status, and 2) it actually increased the interpersonal distance between man and woman if the man was already involved with another woman, showing that the effect of the hormone was influenced by an external factor (the relationship status).

I would be curious so see the same study conducted on women: how would oxytocin and relationship status affect the distance women keep from unfamiliar men?


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