In our series on Big Science Questions, David Buss finds that a lust for love is a universal trait.

Why someone falls in love with one person rather than another from the thousands of potential partners they encounter remains a mystery of profound proportions. Chance, chemistry and the odds that two small windows of receptivity will open simultaneously at the precise moment of meeting guarantee unpredictability. Nevertheless, science has made modest inroads into understanding the ins and outs of love.

Evidence points to love being a cross-cultural universal. In a survey of 168 diverse cultures, anthropologist Bill Jankowiak found strong evidence for the presence of romantic love in nearly 90 per cent. Many people worldwide also report being in love. Sociologist Sue Sprecher and her co-authors interviewed 1,667 women and men in Russia, Japan and the United States. They found that 61 per cent of the Russian men and 73 per cent of the Russian women reported being in love. Comparable figures for the Japanese were 41 per cent of the men and 63 per cent of the women. Among Americans, the figures were 53 per cent of the men and 63 per cent of the women.

My study of the mate preferences of 10,047 individuals from 37 different cultures located on six continents and five islands also revealed the importance and universality of love. I found that "love and mutual attraction" was rated as the most indispensable of 18 possible attributes in a potential marriage partner - by both sexes, in all cultures. Through the vagaries of cultural prescriptions and political regimes, diversity of mating systems, disparities of economic conditions and multiplicity of religious exhortations, humans everywhere apparently long for love.

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The essential qualities people desire in a mate determine whom we are attracted to and which tactics are effective in attracting them. Violations of desire create conflict and predict conjugal dissolution. Satisfying another's desire becomes an effective means to gain and retain a mate. Fulfilment of desire increases the odds of long-term love.

The 37-culture study illuminated more precisely than before what these components of desire are. People the world over want mates who are kind, understanding, intelligent, dependable, emotionally stable, easygoing, attractive and healthy. But cultures differ tremendously on the importance they place on some qualities. Virginity, for example, was judged to be virtually indispensable in a potential mate by most mainland Chinese, but irrelevant by most Swedes and Dutch.

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What was surprising to social scientists was the discovery of universal sex differences. Men worldwide place more importance on youth and physical attractiveness, qualities now known to be important signals of a woman's fertility and future reproductive potential. Women across the globe want men who are ambitious, enjoy decent social status, possess resources or the potential to acquire them, and who were born a few years before they were.

Over human evolutionary history, a woman's children survived and thrived better by selecting a resourceful man who committed to her.

But is love a cold-blooded appraisal of a person's spec sheet or an emotion that blinds us to deficits? It is a bit of both. While people rarely fall in love with those who lack the qualities that they desire, love may have evolved to blind us to a partner's deficiencies. Few people possess the full complement of desired qualities and most of us have to settle for less than we would want in an ideal world.

Usually, only those high in desirability can attract others comparably high. Perhaps the most scientifically documented law of love is assortative mating, the pervasive coupling of people who are similar to each other. Intelligent, educated people marry those who share their insights and erudition. The glamourous pair off with the glamourous. Although opposites occasionally attract, when it comes to long-term love the "eight out of tens" typically marry other eights, while the "six out of tens" go with the other "sixes".

It would not pay to harp on about deficiencies while falling in love. In fact, a recent study reported that most people show "love's illusion" of being overly optimistic about their chances of marital success. Whereas about 50 per cent of marriages will end in divorce, only 11 per cent of married people thought that their own marriage would end in divorce. Among a younger group of unmarried individuals, only 12 per cent thought that their future marriage would have a 50 per cent chance of splitting. These findings may reflect adaptive biases that, although clearly off target, do function to increase the odds of success. According to this explanation, love is an emotion that motivates people to persevere through thick and thin, even if it does not always work out in the end.

In short, love can blind us in two ways - first, by making us happy to settle for someone who is less than our imagined ideal, and second, by being optimistic about the future of the romance, thereby enhancing its chances.

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Evolutionary economist Robert Frank argues that love is a solution to the problem of commitment. If a partner chooses you for rational reasons, he or she might leave you for the same rational reasons, finding someone slightly more desirable on all of the "rational" criteria. This creates a commitment problem: how can you be sure that a person will stick with you? If your partner is blinded by an uncontrollable love that cannot be helped and cannot be chosen, a love for only you and no other, then commitment will not waver.

It is likely that the causal arrow also runs in reverse. Love may be the psychological reward we experience when the problem of commitment is successfully being solved. It is a mind-body opium that signals that the adaptive problems of mate selection, sexual congress, devotion and loyalty have met with triumph. The scientific explanation is that evolution has installed in the human brain reward mechanisms that keep us performing activities that lead to successful reproduction. The down side is that the drug wears off. Some get on a hedonic treadmill, chasing the high that accompanies love. Repeating successful mating with fresh partners brings back the buzz, but perhaps never to its former level.

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Love may be a solution to the commitment problem or an intoxicating reward for successfully solving it, or both. But there is no question that love is an emotion intimately linked with commitment.

In my studies of 115 different actions that signal whether a person is truly in love, acts of commitment topped the list, such as talking about marriage or expressing a desire to raise a family. The most salient acts of love signal the commitment of sexual, economic, emotional and genetic resources to one person.

Unfortunately that is not the end of the evolutionary story. Once desire for love exists, it can be manipulated. Men deceive women about the depth of their loving feelings to gain short-term sexual access. Women, in turn, have co-evolved defences against being sexually exploited, for example, by imposing a longer courtship process prior to consenting to sex, attempting to detect deception and evolving superior ability to decode non-verbal signals.

Another problem is that people fall out of love as crushingly as they fall in it. We cannot predict with certainty who will fall out of love, but recent studies provide some clues. Just as the fulfilment of desire looms large when falling in love, violations of desire portend conflict. A man chosen in part for his kindness and drive may get dumped when he turns cruel and lazy. A woman chosen in part for her youth and beauty may lose out when a newer model beckons her partner.

Then we must consider the harsh metric of the mating market. Consider an entry-level professional couple. If the woman's career skyrockets and the man gets fired, it puts a strain on both because their market values now differ. To the woman, a "nine" who was previously out of reach now becomes available. In the evolutionary jungle of mating, we may admire a woman who stands by her loser husband. But those who do are not like our ancestors. Modern humans descended from those who traded up when the increment was sufficient to outweigh the manifold costs people experience as a consequence of breaking up.

Falling out of love has many dark sides. The crash can be physically dangerous for women and psychologically traumatic for both sexes. Men who get rejected by the woman with whom they are in love often abuse them emotionally and sometimes physically. In our recent studies, we found that an alarming number of men who are unceremoniously dumped begin to have homicidal fantasies. Just as evolution has installed reward mechanisms that flood us with pleasure when we successfully mate, it may have also equipped us with mechanisms that deliver psychological pain when we experience mating failure.

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David M. Buss is professor of psychology at the University of Texas.

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