The old saying "Opposites are attracted to each other" seems to be now scientifically proven.

Because DNA experiments seem to enhance the idea that the long lasting happiness of a couple is linked to their biochemical difference.

A team at the University of New Mexico in Albuquerque, US - led by psychologist Christine Garver-Apgar - researched how genetic similarities among couples were connected to faithfulness and sexual responsiveness to their partners.

The researchers checked genes of the histocompatibility complex (MHC), that encode the tags used by the immune cells to differentiate between pathogens and cells belonging to the body. MHC are known to have an important role in sexual attraction, but their function in a long-term relationship had not been studied. "There have been a number of studies that tested whether MHC similarities played a role in attraction, some involving sweaty T-shirts that women were actually asked to smell," said Garver-Apgar. "What we wanted to know was whether or not these discoveries had any implications for romantically involved couples."

48 heterosexual couples, having an exclusive relationship for at least two years, were asked a series of questions about their relationship, including their sexual responsiveness towards their partner and their unfaithfulness in the relationship.

Their MHC were compared and it resulted that, as MHC genetic similarities increased, women were the most dramatically affected. "They were less sexually responsive to their partners, more likely to have affairs, and more attracted to other males, particularly during fertile days of their menstrual cycles," said Garver-Apgar.

When MHC genetic differences were significant between partners, cheating behaviors were either absent or greatly reduced. MHC sharing and the woman's number of adulterous partners were directly linked: when the partners had 50% of the MHC genes in common, the women had a 50% chance of cheating them with another man. But men appeared entirely indifferent to the partner's genetics.

Men revealed no change in the sexual behavior linked to the MHC of their partners. The scientists suspect this human behavior has a basis in the reproductive necessity to produce offspring with a broad spectrum of immunity. "We're fairly certain that all of this revolves around scent", she adds. "Now all we have to do is track down the specific chemical cocktail responsible for all the behaviors we are seeing."