Why do some couples divorce after one week, while others celebrate their 'golden wedding'. Sex drive 'pushes' people closer to one another, but it's more than the simple biological and behavioral factors than keep them together.

Like begets like. This is the folk perception of the fact that husbands in many cases look like brothers. And genetic researches have revealed that opposites do not attract.

J. Philippe Rushton, a psychologist at the University of Western Ontario, found that heritable personality traits can make 34 % of the decision for friendship and mate selection. "The main theory is that some genes work well in combination with each other," Rushton told LiveScience. "If these genes evolved to work in combination, then you don't want to break that up too much for your offspring. Finding a mate with similar genes will help you ensure this."

A genetically similar spouse means more chances for a happy marriage, and child abuse levels were found lower when genetic similarity is high between spouses. In the end, a parent sacrifices more for a child who is his copy.

Even if women were found to be attracted by men more genetically dissimilar during the fertility peak periods, it is obvious that people are drawn to individuals with the same attitudes and values, essential during the initial attraction. For long commitments, likeness of personality, which can require more time to assess, is more important, as found by psychologist Eva Klohnen at the University of Iowa found in a 2005 study of newlywed couples. "Comedy can also help a relationship. But the importance of humor is different for men and women", said Eric Bressler of McMaster University.

He found that women find attractive the men who make them laugh, but men enjoy the company of women who laugh at their jokes.

But, in the end, what drives a relationship: sex or love? Brain scanning techniques revealed that in the case of people who had recently fallen in love, brain areas connected to love are more active than the sexual ones. "Romantic love is one of the most powerful of all human experiences. It is definitely more powerful than the sex drive," said Helen Fisher, an anthropologist at Rutgers University.

But the main factor maintaining a long-term relationship seems to be fidelity. Men and women value it alike.

A team at the Cornell University led by Stephen Emlen asked about 1,000 people aged 18 to 24 to rate some attributes, like physical attractiveness, health, social status, ambition, and faithfulness, on the importance they give to each characteristic for a long term relationship. People involved in long-term relationships were more precise about what they appreciated in potential mates. Fidelity came fist, followed by physical appearance, family commitment, and wealth and status. "Good parenting, devotion, and sexual fidelity-that's what people say they're looking for in a long-term relationship", Emlen said.