Innate, page 21
Another theory proposes that genius emerges from a qualitative change associated with particular combinations of genetic variants, which, when separated from each other in relatives, do not have such potent effects. That idea is possible, and certainly those kinds of nonlinear, nonadditive interactions between multiple genetic variants can indeed lead to larger differences than one would expect from the simple sum of the effects of the individual variants involved. Such nonadditive effects may be especially important at the extremes of quantitative traits.
Unfortunately, that theory is almost untestable. What we would need to know is whether the identical twins of such exceptional people also showed the same kind of genius. Would Alfred Einstein have been as insightful and intellectually creative as his twin Albert? Would James von Neumann have matched his twin John’s accomplishments? We’ll never know because Alfred and James did not exist. The genetic theory suggests they would have been, but another alternative is that the brains of Albert and John developed the way they did more by chance than genomic design. Any developing dynamic system that has nonlinear interactions will sometimes, very rarely, show transitions into qualitatively distinct states due simply to noise in the system. Most of the time the noise across multiple components and subsystems will cancel out, but there can be very rare occasions that arise, simply by chance, where the noise pushes some combination of parameters into a configuration that leads to a quite distinct outcome. Regrettably, that idea is also effectively untestable and, unless human cloning really takes off, we may never know if there is any truth in it.
1Quoted in Gregory Bergman, The Little Book of Bathroom Philosophy (Gloucester, MA: Fair Winds, 2004), 137.
CHAPTER 9
LADIES AND GENTLEMEN, BOYS AND GIRLS
Are men and women really that different? Obviously, physically they are, but behaviorally, psychologically? Well, if they weren’t, a lot of stand-up comedians would have to go looking for new material. Men and women clearly do behave differently in many ways—on average, at least. The real question is how do they get that way? If we only consider humans in isolation, it can be extremely difficult to dissociate the influence of biological differences from those due to cultural norms and expectations—indeed, these two forces clearly interact in influencing patterns of behavior. But we did not spring, as a species, fully formed from the head of Zeus. We are evolved animals, with a genetic heritage honed over millions of years to ensure the survival of all our ancestors—hominid, primate, simian, and so on, back to the earliest animals. We can thus approach this question from a different angle, examining the biological basis of sexual differentiation and sexual behavior in other mammals, before considering the important effects of culture in humans.
And we can start with the most basic question of all: Why do we have sex? Not “have sex” in that sense—I mean why does sex, as in sexual reproduction, exist? It doesn’t have to. It’s quite possible to reproduce asexually—lots of organisms do it. We could be budding off little clones the whole time, without having to go to all the trouble of finding a mate. We could, but, besides being decidedly unromantic, there are several problems with asexual reproduction.
Foremost among those is that mutations accumulate over time in each clonal lineage. The only way to get rid of them is for individual lineages to die out. That works okay for small, rapidly dividing creatures like bacteria because they can produce so many individuals, but isn’t a good strategy for larger organisms, where it takes a lot more resources to make a new individual.
In addition, because mixing of genetic material between individuals does not occur, it limits the generation of genetic diversity to the random sequences of mutations that arise in each lineage. This means a lot of the possible genetic “space”—all the possible combinations of genetic variants—remains unexplored, which lowers possibilities for adaptation. This lack of diversity also leaves whole clonal populations vulnerable to new infectious agents or to changes in the environment.
Sexual reproduction gets around those problems by mixing the DNA of two individuals every time a new individual is created. This isn’t as easy as it sounds, however. You can’t just smush two cells together. It requires some complicated machinery for one cell to fuse with another and for their genomes to be combined—machinery that we see in specialized germ cells. These cells are also special in that they each contain only one copy of each chromosome, instead of the normal two, so that when they fuse, the resultant organism will have two copies again.
But since you don’t want two cells of one individual fusing with each other (i.e., self-fertilization, which would defeat the purpose), these germ cells come in two varieties, sperm and eggs. Sperm can’t fuse with other sperm, and eggs can’t fuse with other eggs. To keep them separated thus requires two different sexes—one that makes only sperm and one that makes only eggs.
Now that has some very interesting ramifications. In multicellular animals, it means that not only is there a difference between the sexes in the differentiation of the germline, there must also be a difference in the reproductive organs—the bits required for getting the sperm and eggs together. Typically, the sperm are the ones that travel, which for mammals means the fertilized egg develops inside the female. That requires a whole other set of anatomical and physiological specializations not needed in males. It also drastically changes the ecological roles that each sex plays.
Inevitably, this leads to behavioral differences between the sexes. The most obvious of these relate to mating itself. Having two sexes means that each individual can only successfully reproduce with a subset of the other individuals in the species. Because mating is energetically costly—both in expended effort and opportunity costs—and also dangerous if there are predators around, systems have evolved to enable recognition of appropriate partners. That means there must be something outwardly different between the two sexes that can be sensed: they must look or sound or smell different. And it means there must be some neural circuitry to detect those differences. And, finally, there must be some mechanism that says what to do with that information—some neural basis for sexual preference. The most efficient solution for that need, and the one that evolution has settled on, is to prewire that preference into the brains of each sex.
Of course, an animal doesn’t necessarily want to mate with just any member of the opposite sex. To give its own genes the best chance to survive and be passed on, it wants to combine them with other genes that don’t carry lots of mutations, so that the offspring are as healthy as possible. Raising young involves a huge investment, so choosing the right mate becomes a crucial decision. In mammals, this is much more true for females, because they make a much larger investment in their young. They have to carry the fetus, which is energetically costly and dangerous, and which also eliminates any further mating opportunities during that time. Males, on the other hand, can go off and inseminate another female as soon as the opportunity arises.
Also, infant mammals have to be nursed, which only females can do, meaning females invest more in the care of their offspring—indeed, in many species, males play no part at all following mating. This means the number of males ready to breed is usually much higher than the number of females, many of whom are either pregnant or already rearing young. In monogamous species, males stick around and contribute to rearing offspring, but even under those circumstances, their investment is lower than that of the females and their options for other matings higher. For all those reasons, it makes evolutionary sense for females to be much choosier in selecting mates than males, and for males to compete for these opportunities. This kind of sexual selection, which acts as a quality check before investing the resources to actually make offspring, can dramatically drive further differentiation of the sexes, both physically, in anatomy and physiology, and behaviorally (which really means in neuroanatomy and neurophysiology).
SEXUAL SELECTION—MAKING THE SEXES DIFFERENT
As first noted by Charles Darwin, sexual selection can act like an escalating arms race, driving some truly bizarre adaptations and behaviors. If females are choosy, hoping to select mates with higher evolutionary fitness, then males become competitive, aiming to show off their relative fitness with everything from ornate and energetically costly displays, like the peacock’s tail, to the more direct route of simply knocking lumps out of each other. This can lead to differences between the sexes in all kinds of nonreproductive behaviors, especially including aggression and violence. Such differences can also be driven by a division of labor within species, with sex differences in ecological roles, such as nurturing offspring, hunting, foraging, defending territory, social grooming, and other activities.
In primates, the lineage from which humans emerged, these forces have led to males having significantly greater body mass, muscle mass, and bone thickness and density. In species that fight with fangs, the canine teeth are also commonly much larger in males. The extent of these differences varies a lot, however, in ways that reflect the mating and child-rearing strategies and social organization in each species. Those that mate for life and in which both parents invest in rearing offspring, like gibbons, tend to have lower levels of sexual dimorphism (physical differences between the sexes in morphology), while those that continually compete for mates—especially for harems of females, as in gorillas—can show enormous differences between males and females.
Early hominid species also showed substantial sexual dimorphism, as do modern humans. Human males are 15%–20% heavier than females, but have about 40% more muscle mass. Men also have thicker skulls, especially in the front, which may reflect the fact that we like to punch each other in the face a lot. Other differences, such as facial hair, could be due to sexual selection (women may find beards sexy), or may act as dominance displays in competition with other men (to avoid ever getting to the punching each other in the face part). Human males are also much more physically aggressive than human females, as we will discuss below.
Sexual selection affects females too, of course—they also compete with each other to attract the best mates and to encourage male fidelity and investment in offspring. This can drive exaggeration of indicators of fitness and fertility. Since females’ reproductive ability declines with age, these indicators include retention of more juvenile characteristics such as more delicate facial features, higher-pitched voices, and reduced body hair. They also include a greater percentage of body fat and its selective distribution on hips, breasts, and buttocks. These fat deposits emerge with sexual maturity and are needed for ovulation, pregnancy, and lactation. They may thus signal fertility, in turn driving a male preference for these traits.
All of these observations are indicative of sexual selection having played an important part in our evolutionary lineage, including in our early human ancestors. These forces have driven differences not just in our bodies but also in our brains and our patterns of behavior. It is thus not only plausible that such innate sex differences would exist in humans (and demonstrable that they do), it is completely implausible that they would not. It would take a particularly virulent form of human exceptionalism to expect that we should differ from every other species of mammal in this regard.
The question is what kinds of traits do these sex differences affect? Sexual preference is the most obvious one—so obvious that we take it for granted, as if it requires no explanation—but we will also explore below effects on aggression, personality traits, interests, cognitive traits, and even sizable differences in susceptibility to neurological and psychiatric disorders. All of these differences have a physical basis. The brains of males and females are literally wired differently, both in neuroanatomy and in neurochemistry. From studies in humans and, especially, in other animals, we now know a lot about how they get that way.
WIRING MALE AND FEMALE BRAINS
Sex determination in mammals starts with the X and Y chromosomes. In addition to 22 other pairs of chromosomes in each cell (called autosomes), mammals also have either 2 X chromosomes, if they are female, or an X and a Y if they are male. The X chromosome is quite large and carries about 2,000 genes spaced out along its length. These are involved in all kinds of functions, just like genes on the autosomes. The Y chromosome is a very different beast. It’s tiny, by comparison, with only about 200 genes, and most (but not all) of these are involved in male-specific functions, especially in making sperm.
Germ cells—sperm and eggs—have only half the genetic complement of normal cells. That is, they carry only a single copy of the genome, while other cells have two copies. In females, that means a single copy of the X chromosome goes into each egg. But in males, either a copy of the X or a copy of the Y gets put into sperm. Fertilized eggs will therefore either inherit two copies of the X or one X and one Y. So, imagine you’re a little fertilized egg (as you were once), sitting there about to develop and trying to figure out if you should turn into a male or a female. You don’t want to get this wrong and you can’t be indecisive about it, either. That difference in the X and Y chromosomes is all you’ve got to go on, but it’s enough to switch development down one route or the other.
Sexual differentiation proceeds in two stages. First, once formed in the early embryo, the cells of the initially indifferent gonads are directly affected by the presence or absence of the Y chromosome—if it’s there, they develop as testes, if not they develop as ovaries. This depends entirely on the presence of one specific gene on the Y chromosome, known as SRY. If this gene is not functional then XY animals (mice, in this case) will develop as females. And if the gene is put somewhere else in the genome—on one of the autosomes—then XX animals that have it will develop as males. No other genes are required to initiate this switch, though many other genes are involved in the subsequent sexual differentiation. These other genes are present in both males and females—they are just regulated differently. The protein encoded by the SRY gene acts as a transcription factor—it regulates the expression of other genes in the cells of the gonads. In males, it switches on a cascade of gene expression that causes the gonads to differentiate as testes. In females, this doesn’t happen and the female pattern of gene expression is turned on instead, leading to the differentiation of ovaries (see figure 9.1).
Figure 9.1 Sex determination in mammals. The initially undifferentiated gonads develop as testes if the Y chromosome is present, and as ovaries otherwise. The testes then produce testosterone, which masculinizes the developing brain. The X and Y chromosomes also contribute directly to brain masculinization and feminization.
Male or female differentiation of the gonads is the primary event in sexual differentiation, but it is followed by the release of male or female hormones from the gonads. In particular, the testes start to produce testosterone and that leads to the secondary sexual differentiation of the rest of the body, including the brain. (Incidentally, many other species use a different mechanism, where it is the number of copies of the X chromosome that initiates the sexual differentiation pathways, independently in each individual cell in the animal, with no role of the Y chromosome and no influence of hormones—more on that later.)
The crucial role of sex hormones has long been appreciated, but exactly how they function took some working out. First, they play two different roles. In adults, they have an activational role: hormones like estrogen and progesterone in females and testosterone in males are involved in regulating all kinds of reproductive behaviors, most notably the estrous cycle in women. Testosterone is also involved in male puberty, and, as is obvious from doping scandals in sports, higher levels lead to increased capacity to build muscle. Both male and female sex hormones also have acute effects on behavior in adult mammals, including humans, especially on sexual drive and receptivity.
But, crucially, those responses differ between males and females exposed to the sex hormones. There is already some underlying difference between male and female brains, which relies on an earlier organizational function of the sex hormones. Studies in many species have shown that this takes place during an early critical period of brain development, when the brain gets either masculinized or feminized.
In rodents, there is a surge of testosterone produced around or just after birth. To test whether this had any permanent effects on brain development, male rats were castrated at that age and their later behavior analyzed. Even when testosterone was administered to them as adults, these male rats displayed a much lower tendency to mount female rats and attempt to mate with them. Conversely, when female rats were injected with testosterone in the first week of life, they later showed male-like tendencies to mount other females and were much less receptive to being mounted by males. Remarkably, if either the castration of males or the administration of testosterone to females was done later in life, after the first week, these permanent effects were not observed. Similar effects have been seen in guinea pigs, monkeys, and other species. This emphasizes the importance of this early critical period of brain development, when it can be either masculinized or feminized.
But there was a surprise coming. When the researchers injected the young female rats with estrogen instead, their behavior was also masculinized—in fact, it was even more effective than testosterone. That didn’t seem to make much sense, and the explanation for it lies in some rather arcane biochemistry of these hormones and the proteins that interact with them. Female fetuses don’t normally make high levels of estrogen before or around birth, and what they do make is mostly bound up by a protein called alpha-fetoprotein, which prevents it from entering the developing brain. Testosterone, on the other hand, is made at high levels by male neonates and does enter the brain. Surprisingly, though, when it gets there it is mostly chemically converted to estrogen. This is done by an enzyme called aromatase, which is expressed at high levels in the brain.
