Why Opposites Attract
Now scientists claim to have found out why.
A study has shown that women are programmed to seek out men whose immune systems are unlike their own.
The greater the difference there is in the genes which are key to the immune system, the more likely a woman will stay faithful.
But if the genes are very similar, a woman is more likely to be sexually dissatisfied with her partner, have a roving eye and even have an affair.
It is thought that seeking out a mate with a different immune system ensures any children the couple have will have the broadest immunity against disease.
The research could even lead to the creation of a DNA test which would show how likely a woman is to remain faithful.
And it could explain why famously ill-matched couples such as bookish playwright Arthur Miller and Hollywood wild-child Marilyn Monroe fall in love.
In the study, U.S. scientists analyzed the genes key to the immune system known as the major histocompatibility complex, or MHC of 48 couples who had been together for at least two years.
They were then asked a series of questions about their relationship, including whether they had been unfaithful and how sexually attracted they were to their partner.
The more similar a woman's immune system genes were to those of her partner, the less sexually attracted to him she was and the more likely she was to cheat, the researchers from the University of Mexico found.