Thanks to science, unfaithful men now have a new excuse -- their infidelity might be down to their genes.

A study has revealed that men who inherit a genetic variant which affects an important attachment hormone are more likely to have weaker relationships and marital problems, and less likely to be married.

Moreover, their wives or girlfriends are also more likely to be less satisfied with them as partners, the study has found.

According to researchers at the Karolinska Institute in Sweden, the "flaw" gene is linked to the bonding chemical vasopressin which plays an important role in social behaviour, pair-bonding and sexual attachment.

"This (gene) may tentatively reflect a lower degree of commitment. (However), this gene is only a small part of the process -- there are many different ingredients to a happy marriage," lead researcher Hasse Walum said.

In fact, they came to the conclusion after carefully investigating three genetic variations in vasopressin receptor and testing a database of 2,186 twins and their partners to determine which types they inherited, 'The Times' reported.

The Swedish team gave each couple a standard test to assess the strength of their relationship, and asked them whether they had experienced a marital crisis or threat of divorce or separation in the past year.

They found the "love rat" effect applied only to men with two copies of the rogue '334' form of the AVPR1A gene. These men said they found it harder to bond with their wife or girlfriend.

They were less likely to be married to their partners, and their wives and girlfriends were also more likely to be dissatisfied with their relationship. All the subjects used in the study had been in a relationship for at least five years.

The results of the study have been published in the 'Proceedings of the National Academy of Sciences' journal.