Oxytocin (pictured) is a molecule endowed of a double function. Whereas in the female reproductive tract it acts as a hormone and causes uterine contractions during labor, in the brain it acts as a neurotransmitter and regulates behaviors such as sexual arousal. According to Dirk Scheele, University of Bonn, Germany and colleagues oxytocin may also strengthen monogamy. This conclusion stems from a study conducted on a group of men engaged in a permanent relationship and subjected to intranasal administration of oxytocin.

Using magnetic resonance imaging, the researchers monitored the brain activity arising in these individuals after looking at a picture of their partner or a picture of other unknown women. When these men received oxytocin, their reward system, a brain region inducing pleasurable effects, was very active. This phenomenon, however, occurred only when they looked at the picture of their own partner and not when they were exposed to pictures of other women. As a consequence, when receiving oxytocin, the subjects of this study perceived their own partner as more attractive than other unknown women. Oxytocin may thus strengthen romantic bonds in men, by enhancing the perceived attractiveness of their partner.

• Oxytocin enhances brain reward system responses in men viewing the face of their female partner,
  D. Scheele, A. Wille, K. M. Kendrick, B. Stoffel-Wagner, B. Becker, O. Gunturkun, W. Maier, R. Hurlemann,
  Proc. Nat. Acad. Sci. 2013, 110, 20308–20313.
  DOI: 10.1073/pnas.1314190110