Of all the topics related to sex, this one is perhaps the most emotionally charged. We have lots of polite ways of handling the topic, and it is illustrative to notice how angry listeners get when someone breaks the rules. We have many deep seated fears about sexual selection precisely because we recognize an inherent truth. No matter how smart, good looking, or talented we are, the list of people who don't want to have sex with us is much longer than the list of people who do.

The main thesis of this chapter is simple. The human brain, contrary to many opinions, is not a survival adaptation. It was the result of rather arbitrary selection by early hominid females. Our big brains are the result of something called Runaway Selection, and because of our complex society and rare sexual characteristics, it has given us the richness of sexual experience we have today.

It's easy to toss out opinions about whether or not beauty is in the eye of the beholder, or whether beautiful people have it better. I'm going to do my best to avoid such opinions and deal with what we can prove. This chapter is but one in the very complicated subject of sexual selection. You might be surprised at how much material is well known in the scientific community. Before we get to that, I want to briefly discuss some of the obstacles to scientific exploration and public understanding of sexual selection.

The first objection that I am likely to encounter will be that of biological reductionism. Human behavior, critics will say, is much too personal and complex to explain with a simple biological truism. Beauty, humor, artistic talent, and other attributes that make us sexy, it will be argued, are too subjective to be hemmed in by science. Beauty is in the eye of the beholder. It's also skin deep. One man's trash is another man's treasure. Love transcends the mere physical. The cliché festival could continue for a long time, but I'll spare you.

Let me make this perfectly clear. As a human being and as a writer, I stand for the wonder and awe of science. I believe that a proper understanding of what is really going on can only enhance our sense of amazement at the individuality, beauty, and mystery of human sexuality. Far from trying to take away anything, I am trying to give you, gentle reader, a new sense of mystery and wonder. If you perceive any attempt to take something away from you, I suggest that it is part of your own bias, not the material I am presenting.

The next objection I wish to tackle is the culture objection. That is, critics will claim that many of our sexual ideas and ideals are enforced upon us by culture. Twiggy in the 60s, Marilyn Monroe in the 50s, and Britney Spears in the 2000s are all cultural icons forced upon us by “our society.” Real sexuality, they will say, is something that is more personal, and there is something wrong with our society because it tries to tell us what we should and shouldn't like. While there is certainly some truth to the claim that the mass media has a huge impact on our daily lives, there is an inherent misconception in this objection.

As we will see later, many of the things we think of as “cultural” are more properly defined as “secondarily sexual.” Art, literature, science, philosophy, and many of the things we think of as high human achievements are actually side effects of sexual selection. Society was never an end goal of natural selection. Natural selection has no goals. More importantly, natural selection cannot predict anything. Society arose not because it would help humans to be more orderly or organized or lawful, but because those individuals and groups who formed multi-group bonds (society) fared better than those who didn't. When we look at society, it is tempting to see it as something removed from humanity, but nothing could not be farther from the truth. Society is humanity, and what society tells us about ourselves is what is true about ourselves, though the truth is often only apparent to those who are clever enough to recognize the science behind human nature!

Before you stop reading, let me clarify that last statement. When society tells us, for example, that we will only be happy if we have perfectly straight, pearly white teeth, I do not mean to say that it is correct for each of us individually. I live a very happy life, and I've never had braces, though my teeth are decidedly British in appearance, and well shy of the toothpaste models' whiteness level. However, society's obsession with perfectly white teeth represents a real truth about our perceptions of attractiveness. Society does tend to pick a few attributes and elevate them to the status of “perfection,” but we must remember that they are simply exaggerating that which is true on a more mundane level. Given the choice, most people would prefer straight white teeth to crooked yellow teeth in a mate.

Rather than pushing something foreign and harmful onto our otherwise open and accepting minds, cultural norms and generalizations are relatively accurate representations of the desires of the average person. Why do all the underwear models have the same kind of figure? Because the catalogs with those models sold the best. Why did they sell the best? Certainly good publicity helps, but in the end, people buy products they like.

The trick to understanding cultural generalizations is remembering that no human likes only one thing. While it's true that each generation tends to become focused on a certain “type,” we must remember that each of these types is pleasing to a large number of eyes. One of the key dynamics in human sexual selection is deviance from the norm. Both males and females like to choose people who are just a little different than what they see every day. Given this, we would expect nothing less than a new “perfect figure” each generation or two. Please remember as you read the rest of this chapter that there is a big difference between generalizations and specifics, and though you may disagree with the current norm for society, there has almost certainly been a time when your favorite figure was “the big thing.”

Survival of the Fittest, or Battle of the Sexes?

When many of us think of evolution, we automatically think of survival of the fittest. One of my main goals is to help you divorce yourself from this notion. Evolution is actually a very complex system, and a creature's ability to survive is only one factor. It turns out, it isn't even that big a factor. When Darwin finished writing The Origin, he virtually abandoned research into common descent and natural selection. In his mind, natural selection was proven well enough that others would take up his work. The real puzzle was a whole set of features common in many animals that had no apparent explanation – ornamentation.

Why do antlers grow to unwieldy sizes? Why do peacocks have such gaudy and elaborate tails? How did birds of paradise ever develop such bizarre markings and courtship rituals? Natural selection, commonly understood as survival of the fittest, simply can't explain such things. It's patently obvious that large, gaudy tails make a peacock more vulnerable to predators. This is not to even mention the energy cost of maintaining and building such an elaborate appendage. If simple survival is the only factor in reproductive success, all animals ought to be as plain and camouflaged as possible, but the opposite appears to be true. The animal kingdom is not just diverse. It's diverse and incredibly colorful.

After more than a hundred years of research, we know a lot more about what's really going on. Survival in the environment is the least of a creature's worries. If you're living in the arctic, you had better have thick fur and a lot of insulation. This is all well and good, but we have to look at it from the opposite direction before things start to make sense. If you are living in the arctic, you will have thick fur and lots of insulation. Why? Because you inherited them from your parents, who would not have survived if they didn't have them. In other words, the very fact that you are alive means that you are adapted to your environment. Species that were not able to survive in their environment are extinct. Whatever species are alive now are surviving their environment just fine, or they will soon be extinct.

The question becomes this. If survival of the fittest is a non-issue, what is the issue? As we learned in another chapter, sexual selection provides a strong defense against parasites, so clearly survival against parasites is a major factor in evolution. However, this has also been largely solved by sexual selection itself. Granted, there will always be diseases, but seldom will they wipe out an entire species. Again, if you are alive now, your parents successfully survived parasites, at least long enough to reproduce.

The one thing that every creature must face, regardless of his parents' survival fitness, is other members of his species. Specifically, each creature must find a mate. This is part of a broader topic known as sexual selection. Darwin thought of sexual selection as separate from natural selection, but now we know that they are two aspects of the same process. In fact, from many points of view, sexual selection is the most dynamic part of natural selection. When we look at the incredible beauty of nature, we are really looking at direct evidence for the fickle nature of sexual selection.

The first question we must answer is why have sexes at all? Wouldn't it be more efficient if everyone was hermaphroditic? That way, the potential number of mates is doubled, and the species ought to reproduce more efficiently. When we look at nature, we do see a small percentage of hermaphroditic animals, and a few that change sex during their lifetime, but for the most part, separate genders is the most popular way to make babies.

The simplest explanation for this is that it is more effective for two sexes to specialize. If you can, imagine an early organism that was just experimenting with sexual reproduction. Some of them reproduced between two identical members. Having just one sex cell, a balance had to be struck between making enough cells to ensure reproduction, and providing enough nutrients in the cells to make strong offspring.

Now, imagine that some individuals began to change slightly. The balance leaned towards numbers, eliminating prenatal resources for sheer volume of sex cells. These individuals would virtually ensure that they would get to mate, but their offspring would be weaker than average. Now, suppose that at the same time, some individuals were leaning farther towards resources, making fewer cells. These would not be able to reproduce indiscriminately, but when they did reproduce, their offspring would be substantially stronger than others.

You can see where this is going. When the numbers met the resources, they would have the most reproductive success, combining the fecundity of one with the resources of the other, and having stronger babies than those who played the average and only produced average babies.

We don't know if this is exactly how it happened. As I've mentioned in another chapter, we simply don't know the exact sequence of events that brought about sexual reproduction. What we do know, however, is that the math works. Sexual selection with two genders allows specialization, and makes generally hardier offspring.

Now, jump forward on our imaginary evolutionary chain. All of the hermaphroditic individuals have been selected out of the population. There are now males and females, where the males have the large number of small sex cell and the females have the small number of large sex cells. (Incidentally, this is the biological definition of male and female. The larger sex cell is the female, regardless of anything else.)

Imagine that males and females mate indiscriminately. This will not affect males very much, for their investment in reproduction is small, and with so many sex cells (sperm) they need not worry about anything other than finding the next mate. The females, on the other hand, are locked into a period of gestation, and can only reproduce a limited number of times. With hundreds of thousands of generations, it is almost impossible not to believe that at some point, some females would develop a rudimentary system of “judging” potential mates. Perhaps it is something as simple as finding an exact fit between sexual organs. This doesn't seem hard to fathom, particularly if the female had enough of a sense of touch to feel something akin to discomfort or pleasure. In any case, it doesn't matter what the first sexual criteria were, only that some females became ever so slightly choosy.

These choosy females, assuming they selected for something beneficial, would reproduce better than the indiscriminate females. As hundreds and then thousands of generations passed, this preference would be passed down through the female lines, and occasionally, a new measure would be added, so that a million, or ten million years later, females would have a long list of things they wanted from males.

While this was happening, males would be adapting, too. For each new bar that the females set, successive generations of males would gradually adapt, picking up the best traits from their fathers, and sometimes improving upon them, making themselves even more attractive as a mate. If our imaginary females developed a penchant for large, strong males, it's easy to see how males would become substantially larger and stronger over just a few generations. If, however, they preferred males who made good after dinner snacks, we can see how this would also propagate through the population quickly. (Think of black widow spiders!)

If this doesn't convince you that females are overwhelmingly likely to evolve into selectors, consider this. A male's genes face an inevitable death sentence. No matter how long a male lives, when he dies, all of his genes disappear forever. When a female mates, it doesn't particularly matter which male she mates with. Half of her genes will survive until the next generation. For a male, it is not so. Even if he successfully mates with a female, fertilization is not guaranteed. Not only might the copulation be unsuccessful, but she might well mate with another male before fertilization. In short, the only thing a male can do is have sex with as many females as possible, as many times as possible. This is no guarantee. It only improves the odds. For females, being choosy is natural, for they are always certain of parentage, and have every reason to select the best possible mate.

Hopefully, you can see that female selection is nearly inevitable and certainly undeniable in sexual selection. The very fact that females have more to lose makes it nearly certain that if any females become choosy, they will be better able to survive. The rest, as they say, is history.

Runaway Selection

Now to the most important fact of sexual selection. Remember that in the beginning, we imagined females choosing something beneficial to the survival of the species, and that trait led to female choosiness being inherited? Now, imagine that a species has survived its environment well, and is experiencing a stable existence. The tendency for females to be choosy will not disappear. We can easily imagine that some females will arbitrarily choose something that doesn't directly affect survival. If, by chance, this trait happens to be carried by strong males, it will be passed down more than others.

This, in a nutshell, is what we see happening all over the animal kingdom. Females have the tendency to be choosy. It's built into their genes at a fundamental level. Often, this choosiness manifests itself in a whimsical way. This is the primary cause for the incredible diversity of life on the planet. It is not the need for simple survival, for bacteria are still, far and away, the best survivors on the planet. No, it is not survival that drives the beauty of nature. In animals, it is female choice.

For the remainder of this chapter, I want you to forget about surviving against the environment. Instead, I want you to focus on a fundamental principle of sexual selection. Males adapt to females, and females adapt to males. The classic example for this is birds' tails. Suppose you have a species of bird in which the males contribute nothing towards childrearing. This means that females can choose any male they want, and genes will be the only variable once mating is done. Now, suppose that for whatever reason, females in this species prefer males with long tails. (For this example, we've assumed that through chance, a significant number of females have developed this tendency. This isn't hard to imagine. It only requires piggybacking with another beneficial trait.)

In the first generation, there will be very choosy females and relatively indiscriminate females. The choosy females will gravitate towards the few males with the longest tails. Since the males have nothing but time on their hands, they're happy to mate with as many females as possible, so the choosy females have no problem finding long-tailed mates. Their offspring will tend to have long tails. The indiscriminate females, on the other hand, take the first offer that comes along. Their offspring will have average tails.

When the next generation gets to mating age, there will be more long tailed birds than in the previous generation. This will have a double effect. First, the average tail length will rise. Those born to average tailed parents will no longer be average, even though their tails are the same length as their parents. They will now be below average. Second, there will be more long tailed specimens around, but among them, only a few, with the very longest of tails, will get the attention of most of the females. Those whose tails would have been long in the previous generation are cursed with mediocrity in this one.

This process would presumably die out if only tail length was being passed down. However, tail length is not nearly the only trait being passed, and males are not the only sex being born. Young females will inherit their mother's preference for long tails. Assuming that each generation, an equal number of males and females is born, each generation will see a reduction in indiscriminate females. If ten offspring are born to indiscriminate females, five of them are males. Since indiscriminate females are equally likely to mate with a long tail as a short tail, and choosy females are very unlikely to mate with the short-tailed males, it is likely that many of these indiscriminate individuals will not reproduce successfully. Within a very short time, the indiscriminate females will be gone from the gene pool.

Note that this system requires a certain kind of behavior from females. If there's an equal number of males and females, the females need to engage in a certain amount of polygyny. In other words, multiple females have to mate with one male. This clearly happens all over the animal kingdom. In fact, strict monogamy is very rare. That is, there are very few species in which mating happens for life, and only with one partner. (If you look around, you will see that humans are anything but monogamous. This will be important later.)

Now that you understand the basics of runaway selection, you must take something else into account. For all intents and purposes, runaway selection is unpredictable. Because of the randomness of genetic drift, recombination, and external factors such as predation, it's impossible to predict which sexual preference among many will become dominant. Suppose that in our initial generation of birds, there are five different sexual attractions in females: Long tails, colorful tails, colorful necks, facial markings, and crest size. There are five females who prefer each one. At the end of the first generation, all five long tail choosers have survived, none of the colorful neck choosers, and between one and three for each of the remaining categories. The next generation will be dominated by females who prefer long tails. There is no logic to this, only chance. If we were to run a similar “experiment” another time, we would be very likely to start another chain of preference instead.

In fact, each generation is just such an experiment. At any point, a species may be split geographically, or may experience some other pressure that leads to runaway selection in some random direction. Eventually, this leads to a whole new species. If you take any species of bird as an example, you can see that most of the differences between varieties are not linked to survival. They are a matter of color and ornamentation. This is one of the most compelling pieces of evidence for the dominance of sexual selection in natural selection.

The last thing you must understand about runaway selection is that it tends to produce large sex differences. The peacock is a perfect example. The peacock's tail is much larger and more colorful than the peahen's. In fact, we see this throughout nature. When the female is the selector (which is most of the time) the males become disproportionately colorful, with substantially more ornamentation than the females.

The Problem of Runaway in Humans

Clever readers have spotted a problem by now. Humans don't have very pronounced sex differences. If the human brain, as I am claiming, was the result of arbitrary female choice followed by runaway selection, we would expect males to be substantially smarter than females. Clearly, this is not so. By all measures, the differences in intellectual capacity between males and females is slight. The differences tend to be linked towards behaviors that would benefit the particular sex. Men are better at maps because they need to go out and hunt. Women are better at recognizing emotion because they have typically spent more time nurturing and raising children.

Like the question of the advent of sexual reproduction, there is probably not one single answer to reconcile the apparent flaw in the theory of the runaway brain. It is likely that it is a combination of several related effects, each exerting some, but not all, of the evolutionary influence necessary to keep humans relatively equal in brainpower.

1. Shared Genes

Technically, this is called “Genetic Correlation Between the Sexes.” In short, males and females share most of the species' genes. In humans, twenty two of our chromosomes are shared. Only the X and Y set is unique to the sexes. In the short term (evolutionarily speaking, of course) sex differences will remain relatively small. In humans, the gene for brain size is not linked to the gene for sex differences, so when a female chooses a male with a big brain, both her female and male children will have slightly bigger brains.

Since evolution is a gradual process, this can go on for quite some time. In the case of traits that don't directly affect survival, it will take a long time for the detrimental effects to be noticeable in females, making them negative selectors. Take tails for instance. At some point in peacock evolution, the benefit of genetic efficiency was overridden by the detriment of females having huge tails. When this happened, females with smaller tails became better at surviving, and so runaway selection reversed itself. In humans, it is clear that brain size has not become a disadvantage for females, so it stands to reason that runaway would continue to “allow” the shared genes for brain size to be shared by both sexes.

2.Overlapping Capacities

For those readers who insist on believing that we are unique among animals because of our intelligence, here's something to make you happy. Unlike many sexual ornaments, the tools for recognizing intelligence are unique. Recognizing a very big tail takes only visual acuity and the ability to compare, both very low on the hierarchy of intelligence. More importantly, a female with a small tail can still recognize a male with a big tail. However, to understand a subtle joke, one must have the intelligence to make the joke oneself.

In a nutshell, the logic goes like this. Because females decided intelligence was attractive, they passed on the preference to their female children. However, those female children who weren't intelligent enough to effectively select intelligent males didn't choose well, and so filtered themselves out of the gene pool. Only those who were themselves intelligent were left.

3. Mutual Choice

As you're probably aware, humans are not quite so simple as many other animals. To suggest that mating is entirely left up to females is to appear horribly naïve. If only the females do the selecting, why do they go through all the trouble of putting on makeup and dieting and buying the latest fashions – preferably fashions that accentuate their breasts, hips, and other sexually attractive regions?

Obviously, humans have evolved a system of at least partially mutual choice. Scientists actually know quite a bit about it, but the fact is, so do you. A lot of the things you know about dating are true when it comes down to scientific observation. Not only that, but there's good reason for all of them!

First, we have to go back to the motivational differences between males and females. Males want to have sex with as many women as possible, and women want to choose the best male possible. There's an added dimension to it, though. Remember from another chapter that concealed ovulation revolutionized sex in humans, and is the most likely cause for the incredible complexity of our society. In fact, most copulations in humans do not result in a pregnancy, even without birth control.

This becomes very important when we realize that sex has become a social institution and an individual bonding mechanism. Though humans are not monogamous in a biological sense, we do establish long term bonds. In primates, long term generally means at least several months of continued sexual activity. The same measure works for humans. Throughout history, when it has not been prohibited by the government, the tendency has been for young humans to engage in short term sexual relationships with multiple partners, and then to choose a long term partner for childbearing.

Not surprisingly, men are relatively indiscriminate when choosing females for immediate sex. There's very little down side for men to have single or short term encounters with as many females as possible. When there is no long term commitment, a single sex act does not ordinarily preclude having sex with other females whenever possible. If you think about it, this behavior describes an awful lot of men, doesn't it?

Where some people go astray is in thinking of medium to long term relationships. Doug Kenrick, a prominent evolutionary psychologist, has convincingly demonstrated that men are as choosy as women when it comes to long term relationships. In short term situations, men are almost exclusively choosy about physical appearance. However, intelligence, kindness, generosity, and other traits become equally important to most men when choosing a long term partner.

Women are most interested in intelligence for one night stands, although their standards are not as high as when they are pursuing a long term commitment. In almost all cases, however, women are not as physically motivated as men. This observation backs up the notion that intelligence is the result of female selection. There are plenty of species in which the females do very well by picking the strongest males. Remembering that our budding brains were hardly bigger than an ape's when females first decided brains were sexy, it is easier to put the whole thing into perspective.

To return to the original point, recall that for females to be able to successfully attract a mate for a long term relationship, she needs to be intelligent, for males value intelligence very highly in the long run. This factor, combined with the previous two, builds a strong explanation for the lack of big sex differences between human males' and females' brains.

So is it Runaway or Not?

In the end analysis, runaway selection doesn't appear sufficient to explain several aspects of the human brain's development.  This is not to say that some runaway selection has not occurred, or that runaway was not the initial impetus for our brain's development.  The truth is that there is still considerable debate in the scientific community as to the exact nature of early hominid evolution.  However, there is another principle, which I believe works very well in conjunction with runaway, to explain how our brains have remained so similar between the sexes, and why they seem to have evolved much slower than runaway would predict.  Fitness indicators are an integral part of sexual selection, and I believe that including the brain as a fitness indicator in a model of human evolution ties together many, or perhaps even most, of the loose ends.  Fitness indicators will be covered in another essay.

Final Thoughts

While this is hardly more than a primer in sexual selection, it will be the launching point for much more. Once we have established the evolutionary motivations behind our behaviors, we will be able to look at our day to day activities and understand much more clearly why we act the way we do. Rather than look down on ourselves for wanting a one night stand or a rebound, we can understand that it's part of our genetic programming. We are only being human.

It's important to remember that I'm not trying to establish any kind of norms. I'm not trying to tell you that you ought to have one nighters, or that you ought to get married and have kids. Such value judgments involve personal goals, and have to be evaluated individually if they are to have individual merit. I do want to caution you against blanket statements about certain kinds of activity. If everybody has a one night stand once in a while, it's not because everybody is corrupt, or is somehow perverting human nature. It's exactly the opposite. They're following their nature.

Once again, I must remind you of the lesson from sugar craving. Our instincts are what they are. They are neither good nor bad. Evolution has programmed us this way because this is the way our genes were most successful at getting us to make more genes. To make a value statement about our behaviors, we must first establish a goal, much like we did with sugar cravings. For instance, if we want to avoid venereal disease, we should either use protection every time or avoid having multiple partners. However, if it is very important to us to form a long term bond with someone who is highly compatible with us sexually, then we are somewhat obliged to have sex with many people. It is not up to me to form anyone else's morality. I only hope to explain the drives behind the behaviors, and maybe to take some of the stigma off of things that religion has beaten us into thinking of as less than human.

The Origin of Virtue, Matt Ridley, Penguin, 1996

The Mating Mind, Geoffrey Miller, Anchor, 2000

The Selfish Gene, Richard Dawkins, Oxford, 1989 (1976)

The Red Queen, Matt Ridley, Harper, 1993

Human Sexuality, Craig A. Hill, Sage, 2008

Why Beautiful People Have More Daughters, Alan S. Miller and Satoshi Kanazawa, Perigee, 2007