Middle-aged men are more likely to have a poor sex life if their wife is close to their friends because it undermines their masculinity, a study has found.
Researchers concluded that the social networks shared by men and their female partners could have a link to erectile dysfunction.
The study, from Cornell University, found that in middle aged and older men, when the woman gets on better with his friends than he does his sex life suffers.
The phenonomon was dubbed "partner betweeness", in which a romantic partner comes between a man and his friends.
Prof Benjamin Cornwell, who led the research, said: "Men who experience partner betweenness in their joint relationships are more likely to have trouble getting or maintaining an erection and are also more likely to experience difficulty achieving orgasm during sex.
The study found partner betweenness undermines men's feelings of autonomy and privacy, which are central to traditional concepts of masculinity.
This can in turn lead to overt conflict or problems with partner satisfaction and attraction.
The authors said there was nothing wrong with the wife organising most of their social activities because females tended to be more organised.
But they added that reducing a man's contact with his friends to the point that a couple only socialised together was not healthy, suggesting that so called "boys nights" could, in fact, be a good thing.
"The key issue is whether it reduces his contact with his friends while it increases hers, for example she alters his social schedule to the point that his contact with his friends increasingly occurs in the context of couple’s dinners," he said.
"A man’s ability to play a round of golf or to have a few drinks with a friend who has only a passing acquaintance to his wife or girlfriend is crucial to preserving some independence in everyday life.
"If he has to bring his wife along every time they meet, or his wife starts monopolising that friend, that’s when problems may arise.’