Book Summary

Sperm Wars by Robin Baker

25 minutes read

Sperm Wars

The Generation Game

Great Uncle Who?

We are descendants of the winners, the people whose sexual strategy paid off.

It will still be the genes of those among us who produce most descendants that will characterise future generations, not the genes of those who produce few or none.

Normal Service

From the viewpoint of reproductive success, people do not have routine sex primarily to produce children

Men's and women's bodies are programmed by their genes to seek sex with their partners at intervals, as a matter of routine, whether their brains can see a good reason for it or not.

Men's bodies are trying to maintain a population of sperm inside their partner. What women's bodies are tyring to do is confuse the man so he hever knows, either consciously or subconsciously, the best time to inseminate her.

First, as a general rule, a woman's body allows sperm to remain fertile for no more that five days after being deposited inside her. Secondly, sperm seem to need about two days inside the female to reach peek fertility. Thirdly, women produce just one egg per menstrual cycle, but this egg dies within a day of being produced by her ovary.

For a man to have any chance of fertilising a woman, he has to inseminate her at least once during the period from five days before she ovulates to about twelve hours afterwards. To have the best chance, which still isn’t very high (about one in three), he must inseminate her about two days before she ovulates.

It is no accident that a woman is not naturally conscious of when she is most fertile. The uncoupling of her conscious mind from her body’s fertility is as important a part of her body’s strategy as all the other elements.

If a man manages to routinely inseminate his partner about every two or three days, he should always have fertile sperm inside her – in which case, his chances of fertilising her egg in any given month will be about one in three.

The Wet Sheet

To understand much of human sexuality, we need to appreciate the beauty of a woman’s mucus and the amazing things she does with it. She has complex requirements of her cervical mucus. On the one hand, it is her last defence against the bacteria and other disease organisms which are forever trying to invade her cervix and womb. On the other hand, she needs it to allow passage to sperm on their way in and to her menstrual flow on its way out. In other words, she needs it to function as a two-way filter.

Topping Up

An average inseminate contains about three hundred million sperm. Of these, the woman will eject about 150 million in the flowback. A few hundred sperm may go straight to the oviducts and about a million may first go into the cervical crypts to form reservoirs, leaving in order to complete their journey to the oviducts over the next five days. In all, about twenty thousand sperm from each inseminate eventually pass through the oviducts. The remainder, those not ejected in the flowback, clutter up the cervical mucus, eventually to be mopped up by white cells or carried by the slow, glacier-like flow of the cervical mucus (Scene 3) back into the vagina.

The man’s challenge is to adjust the number of sperm he introduces according to the number needed to top up his partner’s reservoirs.


Sperm may rest in an oviduct for as long as a day and, at any one time, up to a few thousand may be resting and waiting. One by one, they wake up and swim further along the oviduct. Their initial destination is a region in which, if an egg is present, fertilisation can occur. But usually no egg is present, and the sperm simply pass through and eventually die.

Sperm Wars

A Chance Affair

A woman is much more likely to have penetrative sex with a man other than her partner during her fertile phase. Moreover, she is much less likely to use or insist on the man using contraception on such occasions.

Once a woman’s body contains sperm from two or more different men, those sperm compete for the prize of fertilising her egg. But the contest that takes place is not a simple game of chance, nor is it just a race. It is indeed a war – a war between two (or more) armies.

In deciding how many sperm to load and ejaculate during routine sex, the man’s body weighs up the chances that his partner might contain sperm from another man. His body does this very simply by registering how much time he has spent with the woman he is about to inseminate since they last had sex.

A sperm army is a much more motley collection of characters than most people expect.

A Sperm War

Each time a killer encounters another sperm, it tests the chemicals on the surface of the other’s head. If those chemicals are the same as on its own head, the killer recognises an ally and moves on to continue its search.

As soon as the newly introduced sperm from the partner attempt to leave the seminal pool, they encounter problems. The channels of the woman’s cervical mucus are nearly all blocked, not only with sperm from the lover, but also with white blood cells from the woman herself.

Conting the Cost

Doesn't He Look Like His Father

The greatest reproductive success is achieved by people who strike the best balance between the pursuit of wealth and status and the production of children.

World wide, it has been calculated from studies of blood groups that about 10 per cent of children are in fact not sired by the man who thinks he is their father.

Making Mistakes

Along with almost all other aspects of human sexuality, the appeal of hard-core pornography owes its existence to behaviour shaped for success in sperm warfare.

Men seek to avoid the costs of their partner’s infidelity by being vigilant for signs, minimising opportunities, and threatening desertion or retaliation. If signs of infidelity are detected, then guarding and threats are escalated. Only if infidelity actually occurs, however, are the threats usually carried out. Even then, a man may decide that his long-term prospects for reproduction are still better if he stays with his current partner than with anyone else – in which case, the threats are curtailed.

Licking Infidelity

Humans are not, of course, the only animals to indulge in oral sex. What all these males are doing is collecting information. They are seeking the answer to three questions. Is the female healthy? Is she fertile? And has she recently had sex with another male? A man is doing exactly the same – and the information he collects can be a big help in his pursuit of reproductive success.

Men may not be able to lick their partner’s infidelity via oral sex, but they can certainly collect information useful for deciding what to do next. In the short term, this information can help men to prepare for sperm warfare. In the medium term, it can help them to adjust the intensity with which they guard their mate or search for another partner. In the long term, it can help them to assess the desirability of desertion and, if their partner produces a child, to judge the probability of the child having been fathered by someone else. At the same time, through strategic prevention or encouragement of oral sex, a woman can attempt to reassure or dupe her partner with regard to the actual situation.

All of these responses were shaped long before there were humans. When our species first evolved, the males simply inherited from our primate ancestors a predisposition for nuzzling, smelling, licking and fingering the female’s genitals. We also, male and female alike, inherited a predisposition to find such behaviour stimulating.


A woman has a lot to lose from her partner’s infidelity.

  1. There is the risk that she will have to share his wealth, time, energy and other resources with the other woman.
  2. There is the danger that he will eventually leave her for the other woman, reducing his support for her still further.
  3. There is the greater risk of contracting sexually transmitted diseases, because her partner is at greater risk.

There is one risk, however, that does not affect her. Unlike a man, she is in no danger of being tricked into raising any children by her partner’s lover. This means that, on balance, infidelity is slightly less of a threat to a woman than it is to a man.

As far as humans are concerned, the best we can use as evidence is that, taking thousands of instances into account, the man is more likely, on average, to have his infidelity discovered than the woman.

At least half of all human infertility is due to sexually transmitted disease.

There is very little variation in the strategy shown by men, women and virtually all other species of animals which form long-term relationships – to attempt secret infidelity on the one hand while preventing a partner from doing the same on the other.

Secret Anticipation

A Double Life

Masturbation between intercourses usually means that a man inseminates his partner with fewer sperm than if he had not masturbated. However, the sperm he introduces are younger, more dynamic, and less hindered by supernumerary geriatric sperm. As a result, just as many, if not more, manage to escape the seminal pool and stay in the woman. Moreover, the sperm which escape are younger and more active – altogether a more efficient army.

In order to have a two-day-old column of sperm waiting to be introduced, a man needs to anticipate his next intercourse with his partner. Subconsciously, the brain must play a major role in this anticipation. The urges that men get to masturbate are timed by their brains and bodies to achieve this gap between masturbation and intercourse.

Multiply, But Don't Divide

Once the brain begins to anticipate infidelity, the body increases the masturbation rate, thereby producing and maintaining a young, killer-rich ejaculate in his tubes, ready and waiting for action. The ideal inseminate for infidelity is one that has been waiting to be ejaculated for no more than about twenty-four hours.

Since the aim of masturbation is to give the male an edge over others in sperm warfare, he gains most if he masturbates but can dissuade those around him from doing so. That way, he gains competitive benefits that his rivals do not. The world-wide tendency to criticise, even victimise, other people for masturbating while continuing to masturbate oneself is thus as strategic as masturbation itself.

The more men drink, the more they want sex but the less able they are to have it. In contrast, the more women drink, the more they want and can have sex. It is just that, beyond a certain point, they may not enjoy it much.

A Wet Dream

Wet dreams can occur at any time of life, but are most common in the teens and early twenties, and are most likely after periods of abstinence from intercourse and masturbation. But only during the teens do they occur often enough to be a direct substitute for other outlets.

Successful Failure

Home for the Day

What seems to happen in each cycle is that in the days after the beginning of a period, a woman’s body goes through a series of hormonal changes. These changes prepare her body to produce an egg but, about one or two days before ovulation can occur, her body goes on hold. Whether she eventually produces an egg or not depends on the events of the following few days, or even weeks. This holding period is an opportunity to collect sperm, perhaps just from her partner, perhaps not – perhaps just from one man, perhaps from two or more. In part, whether or not she ovulates will depend on how her body feels about the man or men from whom she has collected sperm. Most of all, however, it will depend on how it feels about trying to produce a baby in the current circumstances.

The Stress of It All

Carrying even one child as well is difficult. Under such circumstances, the greatest reproductive success is achieved by those who avoid having another child until the previous one can not only walk but can keep up.

The best method of avoiding conception is, of course, sexual abstinence, and the woman and her partner did indeed have spells without sex in their most stressful days. This happened not because they consciously wanted to avoid conception – on the contrary – but because they lost interest in each other. At times, they even felt hostile. Their bodies were manipulating their emotions to reduce the chances of conception. Rarely, however, did they give up routine sex for long. This is because abstinence is in general disadvantageous as an overall contraception strategy - for the following reasons.

Post-natal depression as an irresistible urge to abandon, abuse or even kill a new baby is widely recognised. So widely, in fact, that many legal systems around the world accept that a woman may not be responsible for her actions in the phase immediately after giving birth.

Just as for women, stress is contraceptive. When men are stressed, they produce much greater numbers of these so-called family-planning sperm. The more family-planning sperm and the fewer egg-getters a man introduces into his partner, the lower the chances that she will conceive. But if she is unfaithful and sperm warfare occurs, her partner’s family-planning sperm can help his conventional killers overcome the other man’s egg-getters.

How Forgetful

A woman is less likely to use contraception when she has sex with somebody other than her partner. And it is not always because the circumstances surrounding the infidelity make the use of contraception difficult.

Women, like most female animals that hide the fertile phase of their menstrual cycle, continue to have intercourse well into pregnancy. This is the final touch by which they can confuse the males around them. If a woman lost interest in sex as soon as she conceived, it would be a clear signal to the males around her that conception had occurred.

When a woman has sex with two men over a short period of time, she has three ways of influencing which of them fathers any child that may result. First, she can have sex with one of the men at a more fertile phase of her menstrual cycle. Secondly, she can retain a larger sperm army from one of the males. Thirdly, she can use modern contraceptive techniques.

Shopping Around for Genes

In choosing a man or men with whom to share her life, a woman has two major issues to consider. On the one hand, she needs a man who can help her raise her children. On the other, she needs genes that in combination with her own will produce attractive, fertile and successful children. The better the environment and the better the assistance, the more fully each child will achieve his or her genetic potential.

Most men in most societies have not the time, the energy nor the resources to help support more than one woman and her children at any one time. Her choice of long-term partner is therefore restricted to those men who are unattached, ready to desert their current partner, or who have so much time, energy or wealth that they can support more than one family.

Studies show that when a woman leaves one partner for another, she invariably moves up the scale to a better compromise.

In all cultures, man prefer women with waists significantly narrower than their buttocks. The explanation is that this shape reflects a good hormone balance, good resistance to disease and good fertility.

In addition to shape, men all over the world also respond strongly to clear eyes, healthy hair and skin, and the shape of the face, particularly its symmetry. Again, these features are strong indicators of health and hence fertility. Men of most cultures also respond to breast size and shape, though actual preferences vary and there is no simple link between the appearance of a woman’s breasts and her ability to lactate and sustain a child.

There is another difference in mate preference between men and women. As long-term partners, women tend to prefer men who are older than themselves. Such men have had more time to prove themselves and more chance to amass the resources a woman will need to sustain any children she may have. Unless he is very wealthy, however, a man older than about fifty becomes less and less attractive to a younger, still fertile, woman – because of the increasing risk that he might die before any children she has with him become independent.

Older women are most likely to target young men for acts of infidelity from within a secure long-term partnership, and are less likely to choose them as long-term partners.

The best prizes go to the people who judge correctly when to continue their search and when to settle for what they can get - if only for the time being.

Average woman less likely to have a boy than a woman paired to a high-status male, but women paired to low-status males and women without a partner at all are more likely to produce a daughter.

Taisteful Display

Any behaviour that has evolved to be sexually exciting usually has repercussions in terms of reproductive success. This is as true for a man ejaculating in a woman’s presence as it is for any other sexually exciting piece of behaviour.

In effect, a man seeks or allows oral sex as a display of health or fidelity. He ejaculates openly as a display of health and potency.

It follows that a woman’s best strategy is occasionally to pick her own moment to give oral sex, or to stimulate open ejaculation. The more unexpected she can make those moments, the more information she is likely to collect.

An Abandoned Selection?

Having selected two or more men (by the criteria we discussed in Scene 18) as suitable genetic fathers for her next child, she calls their sperm to battle. This ensures that her child will inherit not only all the other qualities she has selected, but also the genes for the production of a competitive ejaculate.

The penis has evolved its size and shape to remove any material that is already present in the woman’s vagina. In humans, it is particularly effective at removing any seminal pool or any as yet unejected flowback that may still be present.

Xenophilia, a preference for strangers, is a powerful factor in a woman’s choice of mate, particularly as a target for infidelity. In this, women are typical primates.

Finger on the Button

A woman feels like an orgasm whenever her body judges it will enhance her reproductive success. When her body judges it will reduce her reproductive success, she feels no such urge.

First, masturbation ... is an effective way of fighting infection.

Secondly, masturbation increases the acidity of the cervical mucus. For a while after masturbation, therefore, and maybe even for days, sperm are less able to swim through the mucus channels and disease organisms are less able to invade and multiply.

Thirdly, masturbation changes the strength of a woman’s cervical filter – most often strengthening it.

Dark Secret

Although all women have sex dreams, not all women have nocturnal orgasms. By the age of twenty, only about 10 per cent have ever experienced a ‘nocturnal’.

The functions of nocturnal orgasms seem to be identical to those of masturbatory ones. Both types help a woman’s body in its battle against infection. Both prepare her vagina for its next intercourse by depositing lubricant. And both strengthen her cervical filter, as long as there are sperm in her cervical crypts.

Another Successful Failure

On average, only just over 60 per cent of routine sex episodes (from the beginning of foreplay to ejection of the flowback) involve the woman having an orgasm. Even when they do, she usually climaxes during foreplay (35 per cent) or post-play (15 per cent), not during intercourse itself. In fact, only 10-20 per cent of routine sex episodes involve the average woman climaxing while the penis is in her vagina.

By not having an orgasm during intercourse, therefore, a woman’s body is in effect saying: ‘Don’t change a thing. The situation in your cervix is as good as it can be. Just let him inseminate you and your cervical filter will do the rest.

Correcting Mistakes

The fact that orgasms during foreplay are more frequent than orgasms during intercourse suggests that a woman is more likely to err on the side of having too weak a filter than too strong. This could be strategic. It is easier to encourage a man to help her climax during foreplay (strengthening her filter) than to help her climax during intercourse (weakening her filter).

The best moment for a woman to climax (or not) during a sexual episode thus varies considerably from occasion to occasion. The best moment for her to climax as far as the man is concerned, though, varies much less.

Putting It All Together

Of all of the weapons used by a woman in her pursuit of reproductive success, her orgasm sequence may well be the most important, particularly when she promotes sperm warfare.

If a man knew exactly when and how often his partner masturbated or had nocturnals during their routine sexual activity, any change in pattern could alert him to the fact that her body might be anticipating infidelity. With this warning, he could then be more vigilant, guard her more intently, and generally make it more difficult for her to collect anybody else’s sperm.

As we have seen time and time again, most of the strategies shown by men and women in relation to ejaculation and orgasm are subconscious – orchestrated by the body via sequences of mood, libido and sensitivity to stimulation.

Practice Makes Quite Good

Arousal, erection and ejaculation are preprogrammed and automatic, but the niceties of sex have to be acquired. If a male is ever to persuade a female to allow insemination, he has to pick up the subtleties of courtship and stimulation. He then has to learn how to copulate quickly and efficiently so as not to miss the opportunities his courtship technique has provided.

A man who is able to arouse a woman and stimulate her to orgasm signals that he does have past experience of other females. This tells her that other women have also found him attractive enough to allow intercourse. The more effectively he stimulates her the more experienced he should be – and hence the greater the number of women who have so far found him to be attractive. Mixing her genes with his, therefore, may produce sons or grandsons who are also attractive to women, hence increasing her reproductive success.

Rough and Tumble

A man’s reproductive success depends to a significant extent on his ability to make the most of one-off opportunities.

Her priorities need to be when and with whom she has one-off sex, rather than how often. Caution and selectivity are of maximum importance.

Of course, women are not always cautious. The primary situation in which a woman might abandon caution, though not usually selectivity, is when she has a one-off opportunity to collect sperm from a particularly desirable man

Once a woman has a long-term partner, the costs of one-off intercourse are reduced as long as her infidelity remains undetected

Once a woman has tested a man’s ability to force himself on her, she need not do it often thereafter. But as in all such tests of male health and ability, even within a relationship a woman gains from occasional reappraisal of her partner.

How to Con

By preventing sperm from entering the vagina, condoms negate a man’s reproductive benefit from intercourse much more than they negate a woman’s.

If she is without a long-term partner, intercourse can help her gain a man’s attention in her search for one. In addition, she can use the intercourse to gauge a man’s sexual competence, potency, and to some extent health and fertility. Casual intercourse can thus be an avenue to protection and financial or other help from a man who she judges might be a suitable long-term partner. If she already has a long-term partner, casual sex with somebody else can provide a ‘reserve’ – a man to move on to if her current relationship breaks up (Scenes 16 and 19). None of these benefits from casual sex require her to conceive.

Best of Both Worlds

Hhomosexual behaviour is not peculiar to humans. Adolescent birds and mammals often show such behaviour. Male monkeys show the same range of homosexual behaviour as men, from mutual caressing and masturbation to anal intercourse.

As far as humans are concerned, homosexual behaviour is shown by only a minority of men – at least in the largest and most industrial societies. In Europe and the United States, for example, only about 6 per cent of men experience any homosexual contact during their lifetime, most often during adolescence.

In all birds and mammals, including men, the vast majority of males who show homosexual behaviour are bisexual.

There is now convincing evidence that homosexual behaviour is inherited. Genetic inheritance is more often via the mother than the father.

Bisexual men have fewer children over their lifetime, but probably have them earlier in life.

The more male partners a bisexual man has during his lifetime, the more female partners he is also likely to have. Since, on average, a bisexual man will inseminate more females over his lifetime than will a heterosexual man, a bisexual man is more likely to have children with different mothers.

There are costs to bisexuality which can negate the benefits. The most important cost of homosexual behaviour is a greater risk of disease. Even before the advent of AIDS, homosexual behaviour brought with it an increased risk of early death from sexually transmitted diseases, such as syphilis. In effect, bisexuals are programmed by their genes to pursue a lifestyle that trades the benefit of an earlier and perhaps greater production of children (with more women) against the risk of an early death.

The Coming of Women

In any given human society, there tend to be about a third to half as many women bisexuals as men.

We know that by the age of twenty a bisexual woman is four times more likely than a heterosexual to have a child, and that even by the age of twenty-five she is still twice as likely.

The Tenth Tonight

The success of prostitution as a reproductive strategy means that most of us will contain the genes of a prostitute among our ancestors. On average, we should each need to go back through our family tree no further than the 1820s (seven generations) before finding an ancestor who was born to a prostitute (assuming, conservatively, that only 1 per cent of the population is conceived by overt prostitutes).

So it is possible that, as we concluded for bisexuality, prostitution is an advantageous strategy only as long as it remains relatively uncommon, at least in societies in which its costs are high.

Men Are All the Same

When sperm warfare is likely, small testes are a major handicap. The sexual strategy that a man does best to pursue, therefore, is dictated to a large extent by the size of his testes.

In short, men with larger testes are programmed to specialise in the pursuit of sperm warfare – warfare which, because of their large sperm armies, they are likely to win. Men with smaller testes, on the other hand, are programmed to specialise in mate guarding, fidelity and the avoidance of sperm warfare – warfare which, because of their small armies, they would be likely to lose.

In between these two extremes of testis size and reproductive strategy lie the majority of men – those with testes of intermediate size. These employ a ‘mixed’ strategy by which they try to strike the best compromise between mate guarding and sperm warfare, but specialise in neither.

Exquisite Confusion

Compared to men, women are incredibly varied in their sexual characteristics. Some women (2-4 per cent) never orgasm; others (5 per cent) have multiple orgasms, scarcely coming down from one climax before building up to the next. Ten per cent never climax during intercourse; another 10 per cent nearly always climax. Some prefer to be totally passive in their build-up to climax during intercourse; others prefer to be more active. Fifty per cent masturbate regularly; 20 per cent never do. Finally, 40 per cent have nocturnal orgasms, whereas others cannot even imagine such an event.

Consequently, no woman should be hindered in her pursuit of reproductive success by the orgasm regime with which she has been programmed – as long as her choice of mate and subsequent behaviour are compatible.

Total Success

Nothing will remove a man’s subconscious urge to have as many children with as many women as his genes and circumstances will allow. Similarly, nothing will prevent a woman from subconsciously trying to collect the best genes and recruit the best support for her children that her genes and circumstances will allow.