The first direct evidence that oxytocin promotes relationship fidelity in humans has been provided by a study that measured the distance men keep between themselves and an attractive woman. The work is documented in The Journal of Neuroscience.
Oxytocin plays a vital role in triggering childbirth and facilitating mothering. It is known to promote bonds between parents and children, and between couples. Previous studies have shown oxytocin increases trust among people but until now it was unclear what role oxytocin played in maintaining monogamous human relationships.
In the new work, led by René Hurlemann of the University of Bonn, heterosexual males received either oxytocin or placebo via a nasal spray. Then, the men were introduced to a female experimenter that they later described as “attractive.” As the experimenter moved toward or away from the study volunteers, the men were asked to indicate when the experimenter was at an “ideal distance” as well as when the experimenter moved to a distance that felt “slightly uncomfortable.”
The results showed that men in committed relationships who were given oxytocin kept a greater distance when approaching or being approached by the woman they found attractive compared with those given a placebo. In contrast, oxytocin had no effect on the distance the single men maintained.
“Because oxytocin is known to increase trust in people, we expected men under the influence of the hormone to allow the female experimenter to come even closer, but the direct opposite happened,” explained Hurlemann.
The researchers also found that the effect of oxytocin on the monogamous men was the same regardless of whether the female experimenter maintained eye contact or averted her gaze, or if the men were the ones approaching or withdrawing from the experimenter. In a separate experiment, the researchers found oxytocin had no effect on the distance men kept between themselves and a male experimenter.
“Previous animal research in prairie voles identified oxytocin as major key for monogamous fidelity in animals,” Hurlemann concluded. “Here, we provide the first evidence that oxytocin may have a similar role for humans.”
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