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NBC News -- The "love hormone" oxytocin may help maintain romantic relationships by prompting men to keep their distance from attractive women, a new study from Germany suggests.
In the study, men in monogamous relationships who were given an oxytocin nasal spray stayed about four to six inches farther away from an attractive, woman they didn't know, compared with men in monogamous relationships who received a placebo.
The oxytocin spray had no effect on the distance that single men chose to keep between themselves and the attractive woman.
The results suggest the hormone promotes fidelity in humans, said study researcher Dr. René Hurlemann, of the University of Bonn. The findings agree with previous research conducted on prairie voles, which suggested the hormone plays a role in pair-bonding.
In humans, oxytocin has been found to promote bonding between parents and children, increase trust, and reduce conflict between couples. And earlier this year, a study found that couples with high levels of oxytocin in the early stages of a relationship were more likely to be together six months later than couples with lower levels of the hormone.
But until now, there has been no evidence that a dose of oxytocin given after a couple gets together contributes to the maintenance of the relationship, the researchers said.
The study involved 57 heterosexual males, about half of whom were in monogamous relationships. After receiving either a dose of oxytocin or placebo, participants were introduced to a female experimenter who they later described as "attractive."
During the encounter, the experimenter moved towards or away from the men, and they were asked to indicate when she was at an "ideal distance" away, as well as when she moved to a distance that felt "slightly uncomfortable."
The effect of oxytocin on the attached men was the same regardless of whether the female experimenter maintained eye contact, or averted her gaze. Oxytocin also had no effect on the men's attitude toward the female experimenter - whether men received the oxytocin or the placebo, they rated her as being equally attractive.
In a separate experiment, the researchers found oxytocin had no effect on the distance men kept between themselves and a male experimenter.
Future studies are needed to determine exactly how oxytocin might act on the brain to affect behavior, the researchers said.
The study was published in The Journal of Neuroscience.