The hormone oxytocin may contribute to the romantic bonds that keep men faithful.

René Hurlemann at the University of Bonn, Germany, and his colleagues used functional magnetic resonance imaging to study the brains of two groups of 20 men. During the scans, one group looked at pictures of their female partners and unfamiliar women assessed by the researchers as being equally attractive, and the other group looked at photos of their partners and familiar women who were not relatives.

All 40 men rated their partners as more attractive than either unfamiliar or familiar women. But men who received nasal puffs of oxytocin before scanning gave higher ratings for their partners than did those who received a placebo. This boost occurred only with partners, and not with familiar women.

Men given oxytocin also had increased signalling in the reward centres of the brain when shown their partner's face, but not when shown a picture of a familiar woman.

Proc. Natl Acad. Sci. USA http://doi.org/p74 (2013)