In South America pregnant women are openly promiscuous and their babies have multiple fathers. In Britain they are unfaithful in secret. Robin Baker explores this flush of libido

A baby with more than one biological father? Impossible. In an age of IVF and the Discovery Channel - when sperm wriggle nightly across our television screens - such a notion seems as crazy as gooseberry bushes and storks. Sperm plus egg equals baby; one biological mother and one biological father. Such are the indisputable facts of life.

But this insight is recent - not until the late 1800s were sperm seen fertilising eggs (in starfish). Even now, we believe we produce such cells only because scientists armed with microscopes, laparoscopes and the like tell us we do. It is not surprising, then, that our "scopeless ancestors" never really guessed the scientific truth.

Aristotle, for example, supported the notion that whole babies originated in the man, perhaps in his brain, before being "seeded" into the woman via the penis and seminal fluid. This hypothesis was still going strong as late as 1700, so that when sperm were first seen down a microscope scientists thought they would see tiny whole humans inside. For seeding supporters, men were the progenitors, women merely incubators.

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Globally, though, chauvinism was even-handed. Other cultures, mainly in Australia, Brazil and Africa, decided babies entered the mother from the environment. While the mother swam in water, for example. Yet others guessed that a new life was formed entirely from the accumulation of menstrual blood. Neither intercourse nor men entered the equation. The mother was everything.

Some people got at least as far as crediting both sexes with a procreational role. Hippocrates decided that although it must be true that menstrual blood was involved in making babies, the material formed only the flesh not the whole body. The remainder, brains and bones, needed seminal fluid to form.

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The merging of blood and semen was also the idea favoured by societies that inhabited the forests of lowland South America. And with impeccable logic they took their theory a stage further in answer to the following conundrum. If semen that enters a mother while a child is developing forms part of her baby's body, what happens when the woman has sex with several men during pregnancy? The answer seemed obvious. All those men must have a share in the biological paternity of that woman's baby. One baby, several biological fathers - a belief still around today.

Anthropologists have studied a number of South American societies with this belief in shared paternity ("partible paternity" in the jargon). These societies believe that the number of fathers a baby has depends on how free the woman was with her favours over the preceding months. The man with whom she had most sex and who thus contributed most semen to the baby's body is the primary father, the second most favoured male its secondary father and so on.

When a woman gives birth among the Bar of Venezuela and Colombia she publicly names all the men with whom she had intercourse during pregnancy. She then sends an envoy, a woman who attended the birth, to tell each of the men "you have a baby". All of these secondary fathers then have recognised obligations to the child. Child Support Agencies around the world can only envy the simplicity - and efficacy - of the system.

In a number of these societies virtually all women are promiscuous and all children have multiple fathers, so it is difficult to carry out any meaningful analysis of how well the system compares with a single-father system. In other societies only some children are credited with multiple fathers and comparison is possible. So far such a study has been carried out on two such societies: the Bar (24 per cent of children with multiple fathers) and the Ach of Paraguay (63 per cent).

In both there is a clear indication that children with multiple fathers have a better start in life than those without. Among the Bar, for example, a pregnant woman who has a lover as well as a husband runs a lower risk of miscarriage and of the baby dying at birth. This is probably because her lover gives her gifts of fish and game during pregnancy, keeping her better fed and healthier.

The resulting child will also have a higher chance of surviving to age 15 years (80 per cent as against 64 per cent) because it will be supported by two fathers. The incentive for a prospective mother is clear: promiscuity can improve her child's prospects. On average, women recruit two or three fathers for each child but some muster more.

At first sight, the example of people like the Bar and Ach seems to provide a clear case of cultural belief driving human behaviour. If a society believes a child can have several biological fathers, women openly take lovers so as to get maximum help in raising their offspring. In contrast, if a society, such as our own, believes a child can have only one biological father, nuclear families are the norm and women stay faithful for fear of sinking the parental ship.

If only the human condition could be made sense of so easily.

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The first problem with such a simple "cultural" interpretation is that lowland societies with shared paternity can have the same concept of marriage - one husband, one wife - as their industrial counterparts.

The second problem is that women in our society differ only in degree, not in kind. Studies at the universities of Vienna by Karl Grammer and Manchester by myself have demonstrated that the modern city-dwelling woman can also show a peak of wanderlust and infidelity around conception.

When the urban woman is at her most fertile (mid-menstrual cycle) she is more likely to dress provocatively when visiting discos, is more likely to reveal bare flesh when on "18-30" jaunts and is more likely to have casual sex. Moreover, the woman most likely to behave with such sexual abandonment is the woman who is not on the pill and who has left her main man at home. And she is less likely to use contraception when having a fling with a lover than when having routine sex with her partner.

The result is that an estimated one in ten children in Europe and the US is not sired by their mother's husband/partner. And in Britain an estimated one in 25 children is conceived while their mother is carrying around sperm from two (or more) men inside her; sperm that battle it out for the prize of fertilising her egg. If Britons believed in merging blood and semen, rather than sperm and egg, at least 4 per cent of children would be blessed with multiple biological fathers. The only real difference between the women of lowland South America and urban Britain is that the former can be more openly unfaithful whereas the latter are usually much more secretive.

Moreover, without a notion of blood and semen in sight, female chimpanzees and many other primates behave just like lowland South Americans and have open sex with multiple males. These females gain from their promiscuity in much the same way as humans. Gibbons and other primates behave just like us and raise offspring within a one-male/one-female nuclear family. Surprise, surprise, though - at around the time of conception a female gibbon can be seen sneaking off when her partner is not looking to have secret sex with a neighbour.

It seems that irrespective of cultural belief, female primates - human or otherwise - are predisposed, presumably hormonally, to a flush of libido and a wandering eye just before, during and after conception. Such is the chemical legacy that the modern woman has inherited from 60 million years of primate evolution. Knowledge and beliefs may marginally constrain this basic behaviour. But what such human edifices seem not to do - and probably never can - is curb the wanderlust completely.

So will the woman of the new millennium enjoy greater freedom for her promiscuity or suffer greater constraints? It depends. The main advantages women have always gained from promiscuity around the time of conception stem from confusing paternity. By giving several males reason to think they might be the - or a - father of her baby, a woman can contrive to recruit greater support and security in raising offspring; security because if one male reneges on fatherhood, another contender might not.

But as we humans gallop into the 21st century we enter an age of wholesale paternity testing and eventually - we hope - fair and efficient child support legislation. The woman of the future will be unable to confuse paternity - but equally the man of the future will be unable to renege on fatherhood.

So what will happen? Will women become less promiscuous because there is less to gain? Or more promiscuous because the danger of being left destitute is also reduced? We shall soon know the answer.

Robin Baker, author of Sperm Wars, was reader in zoology, University of Manchester. His next book, Sex in the Future, will be published by Macmillan in March.

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