Four years is how long the human pair bond is designed to last, according to American anthropologist Helen Fisher. Lucy Hodges reports.

Forget the seven-year itch. Most divorces occur after an average of only four years, according to Helen Fisher, the American anthropologist whose book, Anatomy of Love, has been translated into 15 languages and won critical acclaim around the world.

Ms Fisher, 49, is the new American guru on love and sexuality. After studying 62 cultures in demographic yearbooks at the United Nations in New York where she lives, she found some basic patterns of divorce. (They have nothing to do with divorce rates.) Combining the new research, which uncovered a four-year itch, with anthropological studies and other material on animal behaviour -- red foxes and eastern robins -- she has woven together an ingenious and convincing new theory of why we fall in love, pair bond, divorce, pair bond again, commit adultery and settle down in middle age.

Her answer is pure Darwin, derived from more than four million years of evolutionary history. Ms Fisher is a sociobiologist, a member of an increasingly fashionable group that believes that genes, rather than the environment, underpin much of human behaviour. When you fall in love, with whom and how you court may be cultural, she says. But how you feel as you are falling in love is biological. "Like pair-bonding in foxes, robins, and many other species that mate only through a breeding season, human pair bonds originally evolved to last only long enough to raise a single dependent child through infancy, the first four years, unless a second child was conceived,'' she explained in her book. "There certainly must have been variations on this theme . . . A host of factors must have affected the length of primitive pair bonds. But across the seasons, as decades turned into centuries, those first hominid forebears who remained together until their child was weaned survived disproportionately, selecting for serial monogamy. The seven-year itch, recast as a four-year human reproductive cycle, may be a biological phenomenon."

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This is Ms Fisher's second book. Her first, The Sex Contract: The Evolution of Human Behaviour, was about why humans marry and was a Book-of-the-Month Club choice. Today she works as a research associate in the department of anthropology at Rutgers University in New Jersey and juggles that job with an adjunct associate professorship at New York University where she teaches human sexuality to graduate students. Neither position could be called well paid. More prestigious is her post as visiting scholar at Phi Beta Kappa, the organisation of distinguished scholars, which enables her to tour the country talking about her work, which has been praised by Harvard biologist E. O. Wilson and by Desmond Morris, author of The Naked Ape.

Why is she not ensconced as full professor in a glittering academic department? The answer is that she made a deliberate choice at an early age to avoid the academic ladder, to write for the largest possible audience, in short, to opt for the difficult life. Her books are written in racy prose. She uses short sentences. Her writing is interesting. For example, she explains the legendary male aversion to intimate talk thus: "Men derive intimacy from playing and watching sports. I am not surprised. What is a football game but a map, a maze, a puzzle, spatial action, and aggressive competition -- all of which engage skills that appeal to the male brain. In fact, watching a football game on television is not very different from sitting behind a bush on the African veldt trying to judge which route the giraffes will take."

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Ms Fisher now has mixed feelings about the decision to go for a popular audience first. "I would probably be in a very different position today if I had published with Harvard University Press," she says. "I don't always go to bed thinking I have done the right thing, but I'm going to keep on trying."

Anatomy of Love took ten years to research and write. The new information in it was the divorce data, particularly the finding that people tend to divorce during and around the fourth year of marriage. "Of course there are all kinds of people who don't do that,'' she says. "What is remarkable is that there is any pattern at all, given how vastly different cultures are, the different beliefs about marriage and procedures for marriage."

She also found that people divorce in their mid-20s, at the height of their reproductive years. Women tend to divorce between 20 and 29 and men between 25 and 29. "That's bizarre,'' she says. "You would think that people would divorce after their children were out of school and they were in middle age, when their reproductive years were over." Ms Fisher found that 39 per cent of divorced people around the world had no children, 26 per cent had one child, and 19 per cent two children. But divorce among couples with three children or more was rare, she discovered, pointing to the fact perhaps that the marriage was working and the species being perpetuated.

Indeed, the older you are, and the more children you have, the more likely you are to stay together. Armed with this new research, she looked at pair bonding in birds and mammals to get an idea of how they behaved. Only 3 per cent of mammals but 90 per cent of birds pair up to rear their young.

Ms Fisher examined some of the species that go in for pair bonding. She found that one of the main reasons was to see their offspring through infancy. "That struck me,'' she says. "All of the songbirds, robins, bluebirds pair up, have eggs, defend their territory and stay together until the fledglings leave the nest. I began to see some pattern in birds and mammals. At the end of the breeding season the pair bonds break up. It is the same with foxes."

She realised that humans had something in common with foxes and robins and other serially monogamous species. The human divorce peak -- around four years -- corresponds to the normal period of human infancy in hunting and gathering societies. Four million years ago our ancestors were roaming the African grasslands in hunter-gatherer bands. The trees provided protection at night but during the day a woman with a small child needed protection and food. Her mate provided this until the child could be weaned from the breast (at about age four for such societies) and looked after by friends and relations. Then the relationship broke up, according to Ms Fisher, just as those of birds and foxes do when their young can stand on their own two or four feet. And the pair would start all over again with different partners. All of which pointed to a pattern with adaptive advantages.

If people have several children with differing mates, they have created genetic variety. They may go through agonies in the process -- and there is evidence that our ancestors went through similar miseries when they split up -- but they are perpetuating the species more successfully in this way than if they had only one mate, Ms Fisher says.

Ms Fisher is not advocating divorce or adultery. "Divorce is a problem,'' she says. "I have done it. It's a horrible thing to do. I'm not trying to tell people how to live their lives differently. I am telling them why they have the problems in the first place."

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Her conclusions are deeply reassuring. Divorce, single parents, remarriage, step parents and blended families are as old as the human animal -- creations of a distant prehistoric age. We get into trouble because we are built that way. Our brain physiology was built to drive and fuel this system of serial pair bonding, according to Ms Fisher. "I think we evolved the stages of love,'' she explains. "Love is primitive.'' The first stage is attraction (what Ms Fisher likes to call infatuation) when we are smitten with giddy, hopeful euphoria about our loved one. "What we are beginning to think is going on in the brain is in the limbic system, the emotional core of the brain,'' says Ms Fisher.

Drawing on the work of psychiatrist Michael Liebowitz of the New York State Psychiatric Institute, she says we feel infatuation when neurons in the limbic system become saturated by phenylethylamine or PEA, which is a natural amphetamine. This stage can last two or three years, or longer, if the loved one lives on the other side of the world or is married to another.

If you are lucky, or sufficiently keen on one another, the attraction turns into attachment, the second phase. This brings a sense of peace and tranquillity. As Emerson said: "Love is strongest in pursuit, friendship in possession." In this stage the opiates of the mind are taking over. The endorphins, which are chemically similar to morphine, are calming the brain and reducing anxiety. This happy connected phase lasts until the feelings of security prove too much for the system.

Then we move into the third stage -- detachment -- when we have too much comfort and crave more excitement. At this point, a relationship breaks up or requires a good deal of repair work. None of which makes for an easy life. Ms Fisher believes the human female is not very well built in this respect. She may be successfully constructed to reproduce but not to conform to her environment or what she wants.

As humans age, however, they calm down. They may still fall in love, namely the middle-aged man promising the moon to the younger women he is seeing. But many such men renege on their promises. They have too much to lose. "I think the brain physiology for love, the architecture of the brain, evolved to ebb and flow to direct our primary reproductive strategy, serial monogamy,'' concludes Ms Fisher. "We're built to pair up, rear our young, break up again and probably also to be adulterous."

Ms Fisher attributes much of her interest in the field to having been an identical twin sister. All her life, she says, she has been trying to figure out how much of her looks and behaviour was down to biology and how much was learned. Fortunately, she was born female. It would have been much more difficult, if not impossible, to say the kind of things she has about divorce and adultery as a man. "In this age, it's easier for a woman to be honest,'' she explains. Political correctness has meant male academics have to be careful about what they say.

Ms Fisher chose not to have children because "you can't do it all''. She grew up in New Canaan, Connecticut, attended NYU and took her PhD at the University of Colorado (her thesis was on when the human female lost her period of heat). "I started out in cultural anthropology, which focuses on why people are different, but I am really interested in why we are all alike,'' she says.

After receiving her doctorate she did not waste much time pursuing the academic route. "I asked how I could make a living without going into academe,'' she explains. "I didn't think I would reach enough people. I didn't want to spend my life with six graduate students."

Ms Fisher had, and still has, a larger agenda. From Colorado she returned to New York where she got a job with Reader's Digest books. Hired as a researcher for a book on American Indians, she lived with the Navaho tribe and honed her writing skills by doing an evening class. When she left Reader's Digest it was to write her first book.

Today she is becoming increasingly interested in the academic world. "I think it's a very powerful educational avenue,'' she says. She is enjoying her teaching, her intellectual discussion with academics and students and her work as a visiting scholar. It is as though she has finally discovered her metier. Her next book is on female sexuality. She is hunting down prostitutes in New York to find out what makes them tick. Her theory is that prostitution is a rational way to make a living. "I don't think women are victims like people think they are,'' she says.

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Anatomy of Love: The Mysteries of Mating, Marriage and Why We Stray, by Helen Fisher, published in Britain by Simon and Schuster.