And it apparently comes in a nasal spray.

PROBLEM: Oxytocin -- a hormone released by the pituitary gland (notably during both orgasm and childbirth) -- is known to affect our behavior. It promotes bonding, sometimes to the extent of making us conformists. Researchers in Germany suspected that a dose of the so-called "love hormone" during a flirtatious encounter with a sexy stranger might cause us to draw in closer, perhaps going so far as to spark a dangerous liaison.

METHODOLOGY: The research team singled out the most attractive female among them to approach their male subjects. Each of the 57 men had been administered either oxytocin or a placebo via nasal spray prior to the encounter. The attractive researcher would stand about 24 inches away from the subject, and then move toward and away from them. The men were asked to determine when the attractive researcher was at an "ideal distance" and when she got too close, making them feel "slightly uncomfortable."

The men confirmed after the experiment was completed that the attractive researcher was, in fact, attractive.

RESULTS: Unexpectedly, the men who had received oxytocin and who were also in monogamous relationships preferred keeping a significantly greater distance between themselves and the temptress researcher -- the hormone promoted bonding with their significant other, not the stranger. They stayed an average of 4 to 6 inches further back than oxytocin-induced singletons or anyone from the placebo group.

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This difference was not observed when the subjects were approached by a male researcher (of undetermined attractiveness), and occurred independently of the amount of eye contact made or whether it was the men or the attractive researcher doing the approaching. 

Neither oxytocin nor relationship status affected the men's perception of how attractive the attractive researcher was.

CONCLUSION: Oxytocin promotes monogamy by preventing men from "signaling romantic interest" to other women. 

IMPLICATION: If you can't get your hands on the nasal spray, there are plenty of ways in which oxytocin release can be stimulated during a monogamous relationship. Sex, yes, but also hand-holding and touch are known to trigger the hormone. However, the researchers warn, "it is clear that for these potential fidelity-enhancing effects to be revealed, female partners would need to evoke its endogenous release immediately before contexts in which the men might encounter other women."

The full study, "Oxytocin Modulates Social Distance between Males and Females" is published in the Journal of Neuroscience .