Why won’t he commit? Why does she inexhaustibly want to talk about the relationship? Why can’t he finish our first conversation before he’s trying to maneuver me into the sack? Why do I have to do so much talking to maneuver her into the sack?

Evolutionary biologists can tell you exactly why. It all comes down the sperm and the egg. Let’s take a closer look at these two troublemakers.

Each man produces one- to three-hundred million sperm per ejaculation, roughly a thousand per heartbeat. Sperm are worthless. Men are free to waste them, squirt them this way and that. Who cares? There’s more where those came from. Half of them come out messed up anyway–broken tails, deformed linings, missing heads. Brainless sperm will try to impregnate a red blood cell like dogs trying to hump your leg. Sperm are not what you would call subtle. There are not many secret mechanisms inside a man designed to gently nurture a sperm. The things just get produced en masse, then fired out. Then we make more

Now consider all the work that goes into producing one egg. An egg is 85,000 times larger, a female is born with all the eggs she will ever have, and it takes on average 29.5 days to nurture one precious egg down its silken passageway.

In fact, virtually everything that goes into making a baby is in the egg. The sperm contributes nothing but genetic material. The rest of the sperm is a delivery system, with a few mitochondria carried along as batteries. Picture a submarine crashing into something the size of San Francisco in order to deliver one pizza. The pizza is all San Francisco needs to build something the size of the Earth. The submarine disintegrates into the fallopian sea.

In purely genetic terms, the investment that a Homo sapiens male makes in the sex act is a courtship and a few minutes of his favorite thing in the whole world. Then he’s free to skedaddle and hopefully impregnate somebody else.

Now think about the genetic investment an Ice Age woman makes in the sex act. She risks nine months of pregnancy, several years of breast-feeding a helpless blob of flesh, then a decade of transforming a rebellious teenager into self-sufficient adult. At the same time, she has to prevent predators from eating her or her offspring, stave off rapists, and gather enough nuts and berries for two. Plus she must provide protein for her baby. It ain’t easy chasing a woolly mammoth with a toddler strapped to your back.

What we have here are different reproductive agendas. Look at this difference Darwinistically. What would be the optimum breeding strategy for a creature who can produce up to 300 million sperm each ejaculation?

Spray them around as generously as possible! Sow the fields with them! Shoot shoot shoot shoot! Hurry! One of these darn things has got to take!

What would be the optimum breeding strategy for a creature who produces one egg a month that, if inseminated, requires a lifetime of labor?

Chose wisely.

Look at bison, birds, apes, your dog. Mostly you see slutty males and picky females. Males are prancing around saying, “Somebody give me a womb! The more the better!” Females are watching the males display and saying, “Give me one worthy male!”

Spend some time with orangutans. The male fights with other males, shows off to display his genetic fitness, and, if he’s lucky, the female chooses him. If she’s not lucky, he rapes her. Then he splits. A male orangutan is incapable of feeling love or loneliness, only seasonal horniness. When a female orangutan sees a male, she knows he’s after one thing. The child-rearing is all up to the female.


So how did we evolve from monkey business to monogamy? How did we evolve trust from so much lust?

Here’s the good news: the amount of male investment in offspring is loosely correlated to the length of childhood dependency. The longer the childhood, the more males evolve to care for their offspring.

When an antelope gives birth, childhood lasts about two minutes. The baby antelope reaches a state of self-sufficiency—meaning it can run like heck from predators– almost immediately. It quickly learns how to munch grass and fend for itself. No male antelope providers needed.

Imagine if that state of dependence lasted a year. The poor female is trying to protect her helpless offspring from predators all that time. Babies are being eaten left and right.

Now suppose a random genetic mutation in the male antelope causes him to hunt down and bring home grass for his little baby antelope. That male antelope is going to have more descendants who survive to adulthood than all those other deadbeat antelopes. His gene for childrearing will steadily spread through the population

Let’s consider primates. You can draw out a graph and roughly correlate the duration of childhood helplessness with paternal investment. Chimpanzee males provide meat for the females and care for the young for several years. Baboon males invest for less time. Ring-tailed lemur males could care less.

Gibbons are monogamous to the point of piety. The Christian Coalition should adopt as their mascot the white-handed gibbon, with his perfect nuclear family of one loyal gibbette and several gibblets.

Which primate has the longest childhood? Homo sapiens. As our brains got bigger, it required early hominid women to give birth earlier and earlier to fit that huge head out her cervix. If we gave birth at a stage of brain development comparable to normal apes, women would have to be pregnant for 18 months. Which would you rather do, ladies, stay pregnant twice as long and give birth to a toddler’s head, or squeeze that kid out a little early? Which will require less Pilates classes to change your waddle back to a sashay?

So we pop ‘em out earlier. Compared to most mammals, human females give birth to a fetus, and it stays helpless a very long time. At the same time, our complicated tribal societies got harder to learn, which favored genes that made childhood longer and longer.

Now look at the state we’re in. Our babies won’t even be able to breed for more than a decade, and they’re not that adept at synthesizing the codes of our increasingly complicated tribes until they’re twenty or so. (Twenty-five for my younger brother.)

As our brains got bigger, we had to be born more and more helpless, and our childhoods got longer and longer. Motherhood became the toughest task in nature.

Meanwhile, there’s all these males running around with no genetic investment in their own potential for pregnancy, so their bodies and brains can focus more optimally on other tasks, like hunting baby antelopes. Overworked females can’t help noticing they have something these antelope providers want: steady nookie.

I ask you, hominid ladies, which male are you going to have sex with? The males who are sweet and bring meat? Or the males who bail and chase tail?

Males who stuck around raised more offspring to adulthood than males who didn’t care. Females who found these loyal males sexy raised more offspring to adulthood than females who found deadbeats sexy. Slowly, dads evolved at the expense of cads.

In many species with long childhoods, female choice bred males to evolve increasing emotional investment in their offspring. Some male birds babysit the eggs while the female is off cuckolding him. Devoted deer mice dads raise children who grow faster and survive more often than children of deer mice single moms who raise children Murphy Brown style. Chimp males offer meat for sex in a kind of courtly prostitution. Homo sapiens males offer diamond rings and drive phallic cars to advertise their ability to provide, and Homo sapiens females in modern foraging societies unashamedly demand meat from their lovers. Despite the sperm and egg problem that created radically different breeding strategies, our prolonged childhoods meant we evolved to form intense attachments to whoever we happen to be boffing.

Biologists call this the pair-bond. We’ve institutionalized it as marriage. The bad news is, our pair-bond is designed to last for as long as it takes our offspring to reach some level of independence. Genes aren’t designed to make us happy. They design us to make more copies of themselves. To last “until death do us part,” sexual relationships must develop the natural bonds of friendship and affinity that we also evolved on the Pleistocene savanna. How lifelong friendship evolved is a subject we will explore later.

But first we have other man/woman problems to tackle, foremost being the differences in what makes male and female hominids horny.

Why do I keep calling you a hominid?

Hominids are all the Neanderthals, Australopithecines, Homo habilis, Homo erectus, etc, the upright-walking apes of which we are the only surviving species. Many of these folks roamed the earth at the same time, met, traded goods, possibly mated, and ate each other. Some biologists, along with Plato, define hominid as “featherless biped.” A hominid is really any extinct ape more closely related to us than the chimpanzee. Our desires, virtues, talents, and demons were constructed by their struggles.

Everything unique about Homo sapiens brains and bodies was designed on the African savanna during the Pleistocene era. The savanna is an open grassland with clumps of trees. The Pleistocene is an epoch that coincides with the advancement and recession of the last Ice Age, from 1.8 million to 10,000 years ago, when hominids ran amok over the planet, made many large animals extinct–including most other species of hominid–and rapidly turned into us.

Hominids are a subset of apes. Apes are subset of primates. There are about 235 species of primates, including us.

Monogamous primates tend to be the ones who live in trees. Primates who came down from the trees to compete for ground territory started conniving to hoard mates and sleep around.

When’s the last time you slept in a tree? How do you feel about someone walking across your lawn?

A British survey showed that 60% of husbands and 40% of wives admit to cheating on their spouses. The Kinsey study found that 50% of men and 26% of women under forty in the USA had extramarital affairs–though half of women who read Cosmopolitan report infidelities. This is no recent breakdown of family values. These rates have shown no significant changes in a century–except that people are cheating earlier in their marriages.

The problem with surveys is they don’t measure how much sleeping around we actually do. They measure how much sleeping around we say we do. Humans are notoriously secretive about infidelity, while some tend to exaggerate their conquests.

When I was in high school, one of my fellow geeks conducted a survey to see how sexually active were our classmates. The surveys were filled out in the bleachers during gym, with much looking over shoulders and giggling. The results were startling. It turned out 99% of boys were having sex with 1% of girls. I was morally outraged! Who were these girls, and why hadn’t I been introduced to them? I was tired of being one of only three male virgins in the entire school. Though I knew the boy figure wasn’t exact, since I had lied. It also turned out I was the only teenager who had ever masturbated. Good thing I lied about that, too. As I scanned my class of Cassanova boys and chaste girls, it dawned on me that the only thing the survey revealed was what we wanted to be true.

Exactly how naughty were we in our ancestral environment? If only prehistoric promiscuity could be precisely measured!

It can.

If you want to know how promiscuous males were on the Pleistocene savanna, look at the size ratio of men to women.

We’ll deal with that later.

If you want to know how promiscuous females were on the Pleistocene savanna, look at the size ratio of the male to his testicles.

Let’s deal with that right now.

[Trivers, R.L. (1972) Parental investment and sexual selection. In Sexual Selection and the Descent of Man (ed. B. Campbell). pp 136-179. Atherton, Chicago.]

[Ridley, Mark. (1978) Paternal care. Animal Behavior 26:904-932.]

[Eibl-Eibesfeldt, Ireanus (1989) Human Ethology. Aldine de Gruyer, New York]

[Ridley, Matt. (1993) The Red Queen: Sex and the Evolution of Human Nature. New York: Macmillan. pp. 171-244.]

[Kinsey, Alfred C. et al. (1948/1998). Sexual Behavior in the Human Male. Philadelphia: W.B. Saunders; Bloomington, IN: Indiana U. Press. (p. 585, 587).]

[Kinsey, Alfred C. et al. (1953/1998). Sexual Behavior in the Human Female. Philadelphia: W.B. Saunders; Bloomington, IN: Indiana U. Press. p.416]

Note: Wouldn’t you know it, some taxonomists are trying to change the word “hominid” to hominin” right when my book is coming out. That’s because chimps, bonobos, humans, and gorillas were found to be more closely related to each other than they are to orangutans, so taxonomists want a whole new category now. But I’m going to stick with the traditional “hominid,” just to annoy them.