It's not paranoia – men in a committed relationship are seen as kind and faithful, and thus much more attractive to single women.
A 2009 study showed that given a choice between singletons and men that were already in a relationship, the female participants didn't hesitate: About 50% were interested in pursuing a relationship with the single men, and a staggering 90% wanted to have a relationship with the man that was already taken.
The phenomenon is known as "mate-choice copying", the action of finding someone more attractive because someone else has selected them. It's a mechanism birds rely on to find the best mates.
But a new study published in Scientific Reports shows that in humans, it's less about the man and more about being influenced by others
While the study confirms some of the previous research findings, it also shows that "copying" other women's choices isn't limited to romantic scenarios. It's not limited to sex orientation and can also apply to objects.
The study surveyed 42 women about 30 times each, asking to rate the level of attractiveness of a pair of hands, a work of art and a man's face.
They would be subjected to just the pictures and asked to rate how attractive they found the hands, the face, the art.
Later on, they would be asked to rate the same pictures of hands, face and arts again, but would be shown what other people thought of them during the first survey (called social rating). The study then compared whether people changed their answer based on what other people thought.
The study found that after seeing other people's response to the images, participants would adapt their own answer so that they'd be 13% closer to the social rating for the man's face. They found they also adapted their ratings for the hands and art so that it was closer to the general consensus.
"Women in our study found men's faces more attractive if other women had given that face high ratings, but the same goes for pictures of abstract artworks," said lead author Catharine Cross, from St Andrews University.
These results show it's more about fitting with the crowd rather than trying to steal anybody's boyfriend: humans value other people's opinions and adapt their own accordingly.
"Women appear to copy the mate preferences of other women, but this might simply be because humans have a general tendency to be influenced by the opinions of others."
"Social influence affects every area of our lives, and this could include partner choice. But there isn't, at the moment, clear experimental evidence of a specialised mate-choice copying mechanism in humans," said Dr Sally Street, from Durham University, who co-wrote the study.