New research suggests that men's tendency to be unfaithful may be influenced by their genes.
According to the study, men who inherit a genetic variant that affects an important attachment hormone are more likely than usual to have weaker relationships and marital discord, and less likely to be married.
Also, their wives and girlfriends are also more likely to be less satisfied with them as partners, reports New Scientist.
The study's authors cautioned that any effect would apply only on average, and that it was impossible to predict whether any individual would be unfaithful or a bad partner on the basis of his genes.
The gene in question affects the receptor for a hormone called vasopressin, which plays an important role in social behaviour, pair-bonding and sexual attachment.
Its effects were first characterised by studies of different species of voles. Although the meadow and prairie voles are close cousins, their sexual behaviour is dramatically different.
Now, in the latest study, Hasse Walum at the Karolinska Institute in Stockholm, Sweden, and colleagues looked at the various forms of the gene coding for a vasopressin receptor in 552 Swedish people, who were all in heterosexual partnerships.
The researchers also investigated the quality of their relationships.
They found that variation in a section of the gene called RS3 334 was linked to how men bond with their partners. Men can have none, one or two copies of the RS3 334 section, and the higher the number of copies, the worse men scored on a measure of pair bonding.
Not only that, men with two copies of RS3 334 were more likely to be unmarried than men with one or none, and if they were married, they were twice as likely to have a marital crisis.
Given that everyone surveyed had been in their relationship for at least five years, the team suggests that having multiple copies somehow contributes to commitment problems in men.
The study has been published in the journal Proceedings of the National Academy of Sciences.