There are few choices that have many important consequences as forging a lasting relationship with a person of the opposite sex. The only comparable choice is the decision of what career we will spend a life working in. What kind of relationship you commit yourself to, and what terms, will determine how healthy you are likely to be for the rest of your life, how happy and satisfied you will feel, and how comfortable you will be financially. All of these desirable outcomes - health, happiness, and financial stability - are statistically more frequent among people who have stable and satisfying relationships.

Marriage is not the only way men and women can live together, but despite the liberalization of sexual mores in the past few generations it is still the case that generally marriage is the most effective way of ordering the relationship between the sexes. For instance, couples who have lived together before marriage are on the whole less satisfied with their relationship after they are married, compared with couples who start living together only after marriage. And despite ideologically based attempts to abolish marriage as in the Soviet Union in the 30's, or to weaken it in favor collective arrangements as in China or the kibbutzim of Israel, the need for an exclusive and stable relationship has driven people back to traditional forms of socially sanctioned union.

On what basis do we choose a person of the opposite sex as a life companion? As is true of most complex human actions, this decision is influenced by several reason - often conflicting with each other. In the first place, it is important to always remember that we humans share with other mammals a set of genetic instructions that direct us to have children who will survive to have children in their turn. If the majority of humans did not obey this impulse, we would not have survived so long.

As our body grows and reach a certain size, a number of hormones are triggered to prepare it for reproduction. At that point boys become attracted to girls and vice-versa. Especially when there is nothing else interesting to think about or to do, sex becomes an obsession that for many years will occupy every free moment of the mind.

Genetic psychologists have shown that this reproductive imperative determines quite strongly much of our sexual behavior. They have also shown that men and women approach relationships differently as a result of their biological makeup. Men are attracted to women who are (or who appear) young and healthy, because this signals that the woman will be able to bear children and take care of them until they grow up. Women are attracted less by men's physical characteristics, and more by their status and wealth which signals their ability to provide for the material needs of the family. Both men and women look for mates who will be faithful to them, but for different reason: Men are concerned least their wives bear another man's child, whom they will have to support; while women are concerned that their husband will pay attention, and divert their resources to another women.

These genetic instructions influence the behavior of any person made of flesh and blood. To deny their importance would be foolish. But we are not only biological organism. Ever since the human brain has evolved to the point of making memory and communication possible, our behavior has increasingly been determined by learning. Therefore cultural influences play an essential part in our choice of partners. Parents, books, clergymen tell us what kind of person is worth having for a friend and spouse. We learn about ideas of romantic love, married love, faithfulness - all rather recent cultural inventions. Some of the advice we get is based on sound experience, and by following it we can build a life worth living. Unfortunately, cultures also contain a great deal of deceptive information. Economic and political interests exploit our biological predilections through promises and commercial advertising - sex becomes a lure to attract our attention and money. Perhaps the most difficult task in growing up to adulthood is to learn to tell apart helpful from deceitful information. If we trusted television and the ads in glossy magazines, we would believe that what made women desirable was hair spray, nail polish, and a starvation diet; and that what made men attractive was an intimidating attitude, a powerful car, and a Wall Street stockbroker.

Every culture ways to channel sexuality into socially acceptable forms. Rules about marriage are some of the most universal and central features of any society. Who can marry whom, and under what conditions, are some of the basic features of social order. In most traditional human groups, only men who had enough cows, corn, or other material resources to pay the bride-price to a woman's family could marry and have children. This seems like a harsh and backward practice to us, but as a way to control population and make sure that all children could grow up without becoming a burden on the community it was an effective solution.

Marriages have also been used since time immemorial to establish and maintain alliances. When a women married a man from another family or tribe, the two kinship groups were drawn together by the union. This was particularly obvious in the case of the marriage of the sons ans daughters of kings. For instance, the Habsburg family that ruled Austria for many centuries was famous for marrying young princes to the right ruling families of Europe, thereby avoiding wars and protecting their interests. It used to be said: "Bella gerant alii, tu felix Austria nube (Let others wage wars, you happy Austria, marry). But the same principle applied to the poorest peasant families as well. Therefore it has generally been the case that the most important consideration in who should marry whom was not the preference of the young couple, but the interest of the family as seen by its oldest and most respected members.

Because the continued existence of any human group depends on the birth of children who will be taken care of their parents, marriage and sexuality in general is a primary concern to society. This concern is usually experienced by young people as an unwarranted intrusion in the most intimate aspect of their lives. In our time, it is taken for granted that one's sexual affairs are nobody's business but one's own. Yet many young people who believe that have a right to have a baby if they want, later expect society to take care and educate their children. As the anthropologist Margaret Mead pointed out a long time ago, it is strange that we require a licence to drive a car, but not for becoming a parent.

Biology and culture tell us who is attractive, and why. But there is a third set of concerns that influences the choice of partners, and that is the most interesting one. As we mature with a unique set of values and experiences, we develop preferences that represent who we genuinely are - not just as biological organism, not just as members of society, but as individuals. Thus we become attracted to specific others who seem to hold the key to our happiness by having some trait that we admire - a Thoughtful look, a liking for a certain movie, a ready laughter. Often these traits are trivial, often they are not genuine, and often even if genuine they are not what we need to be happy. But they seem to us at the time the height of human accomplishment, and we fall in love with whoever has them.

According to historians, romantic love is a very recent invention of the human race, whose origins can be dated to about seven or eight centuries ago in Western Europe. Of course people must have experienced the feelings involved in love for millions of years. But the idea that a man and women could love each other completely - in a physical, emotional, intellectual and spiritual sense - seems to appear slowly in human history. Even the Greeks and Romans conceived of love in terms that are rather primitive by our standards. If nothing else, this indicates that we have evolved not only technologically, but have made at least some little progress psychologically as well.

But romantic love is quite fragile. The initial attraction usually fades quickly. The existing becomes routine. The great promise looks like it will never be fulfilled after all. Each partner soon starts to withdraw in his and her own self-centered habits, and the vicious circle becomes rapidly more centrifugal.

Like every other human arrangement, a couple in love is exposed to the force of entropy that breaks down whatever tries to stand out of random chaos. To keep the relationship vital and meaningful, we need to cultivate it, to invest energy in it. If both partners are only trying to get out of the relationship what they want for themselves, the couple will break down and become what it was before: two selfish individuals each going its own way. The force that transform a couple into more than the sum of its parts is a love that goes beyond self-interest. But going beyond self-interest does not mean denying self-interest. Rather, it means discovering that the self can be a limiting entity, and that by investing energy into something greater - in this case, the love for another person - the self will actually expand.

Few people have the inner resources to accomplish this transformation on their own. Those who cannot, usually end up leading restricted, solitary lives. Those who can are helped either by innate wisdom, by luck, or by religious or philosophical advice. Can psychology contribute anything useful? My studies and experience suggest that it helps to think of relationships as a voyage of discovery into a fascinating new world. If we approach it with the expectation that our partner is also a unique being from whom we can learn new ways of seeing wold, then the relationship is likely to stay fresh and enjoyable. Of course that implies being willing to grow, to change habits, to pay attention to one's partner, and to sacrifice one's wishes for his or her sake. If we think of marriage as joyous adventure, and we are willing to work for it as we would for any real adventure, then the relationship will endure after the hormones have stopped flowing, and the societal pressure have lifted. It will continue because we enjoy it, because it is the most fulfilling way we can live our lives.