Dr. Rene Hurlemann of the University of Bonn in Germany said oxytocin, produced in the hypothalamus part of the brain, is also involved in the formation of social bonds. In humans and other animals, this brain chemical is known to promote bonds between parents and children, and between couples.
The study, published in The Journal of Neuroscience, found men in committed relationships who were given oxytocin kept a greater distance when approaching or being approached by an unknown woman they found attractive compared with those given a placebo. However, oxytocin had no effect on single men.
The researchers administered oxytocin or placebo via a nasal spray to a group of healthy, heterosexual males. Forty-five minutes later, 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," Hurlemann said.
"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," Hurlemann said.