Testosterone Rex

by Cordelia Fine

  This is exactly what Joel found for the very first time in humans, with colleagues from Tel Aviv University, the Max Planck Institute, and the University of Zurich.29 They analysed images of more than 1,400 human brains, drawn from large data sets from four different sources. First, they identified around ten of the largest sex differences in each sample. Even this preliminary exercise challenged popular understanding in a couple of ways. First of all, contrary to the view that the brains of men and women are strikingly different, none of these differences were particularly substantial. Even for the very largest, the overlap between the sexes meant that about one in five women were more “male-like” than the average male. What’s more, each data set had a different Top Ten list. As the authors point out, this shows that sex differences in the brain aren’t simply due to sex, but depend on additional factors, the most obvious candidates being age, environment, and genetic variation.

  Next, the researchers identified a “male-end” zone and a “female-end” zone for each brain feature, based on the scores of the 33 per cent most extreme men and women, respectively. (An “intermediate” zone lies between the two.) Then they worked out whether people’s brains are consistently on the male-end or the female-end of the continuum in each of these regions, or whether brains are a mix of male-end and female-end characteristics.

  As the rat data might already have led you to expect, the results were decidedly in the favour of a mix. Between 23 and 53 per cent of individuals had brains with both male-end and female-end features (depending on the sample, type of brain measure, and method of data analysis). The percentage of people with only female-end or only male-end brain features was small, ranging from 0 to 8 per cent.30

  So what is a “female brain” or a “male brain”?31 Is a female brain the type of brain possessed by the very few individuals with consistently female-end brain characteristics—some of whom, by the way, are men? And if so, what kind of a brain do the majority of females have?

  So sex does indeed matter, but in a complicated and unpredictable way. Although there are sex effects that create differences in the brain, sex isn’t the basic, determining factor in brain development that it is for the reproductive system. Unlike the genitals, “human brains cannot be categorized into two distinct classes: male brain/female brain,” Joel and colleagues conclude. Instead, they are “comprised of unique ‘mosaics’ of features.”32 One way to think of it is like this: a neuroscientist certainly might be able to correctly guess your sex from your brain, but she wouldn’t be able to guess the structure of your brain from your sex.33

  There is another important difference between sex differences in the genitals and sex differences in the brain. When it comes to the former, these very obviously serve males’ and females’ different fixed, timeless, and universal roles. Not even the most determined advocate of the view that a woman can do anything a man can would deny that a penis and testes function much better for delivering sperm than does a vagina and ovaries. But when it comes to the brain, “in many cases… the functions of neural sex differences are mysterious,” point out neuroscientists Geert de Vries and Nancy Forger,34 particularly the farther “up” the brain you get, away from the parts of the brain involved in very sex-specific functions, like ejaculation. Summarizing decades of endeavour in 2009, de Vries and a colleague note that

  hundreds of sex differences have been found in the central nervous system, but only a handful can be clearly linked to sex differences in behavior, the best examples are found in the spinal cord… we do not know the functional consequences of most of the others.35

  This might come as a shock to some, especially given the willingness of some scientists and popular writers to conjecture links between sex differences in the human brain and complicated, multifaceted behaviours like mathematics, empathizing, or taking care of children.36 But these speculations are, to put it politely, optimistic. There are no simple links between a specific brain characteristic and a particular way of behaving. Instead, how we think, feel, and act is always the product of complex assemblies of neural effort, in which many different factors act and interact.

  To be very clear, the point is not that the brain is asexual, or that we shouldn’t study sex effects in the brain. (Just for the record, I’ve never held that view.)37 As several neuroscientists have argued, since genetic and hormonal differences between the sexes can influence brain development and function at every level (and throughout the brain, rather than just in a few reproduction-related circuits), investigating and understanding these processes may be especially critical for understanding why one sex can be more vulnerable than the other to certain pathologies of brain or mind. This, in turn, may offer helpful clues to potential causes and cures.38 The point is rather that, potentially, even quite marked sex differences in the brain may have little consequence for behaviour.

  This may seem counter-intuitive. In a keynote speech titled “When Does a Difference Make a Difference?” University of Toronto neuroscientist Gillian Einstein reflects on her puzzlement when she encountered this situation in her own research.39 On the one hand, as she explains, there’s clear evidence that oestrogens and progester-ones can powerfully affect the growth, pruning, and connectivity of brains cells. Yet meticulous work in her lab identified little correspondence between oestrogen or progesterone levels (over the course of the menstrual cycle) on healthy women’s moods—negative or positive. Contrary to popular myth and a thousand misogynist jokes—paging Mr. Donald Trump40—the important predictors of mood aren’t time of the month, but stress, social support, and physical health.41 Einstein had “a hard time with this,” as she puts it. Intuitively, it stands to reason that “if you affect neurons, you affect the brain, and if you affect the brain, you affect mental states.” Yet this wasn’t what she found. Einstein’s conclusion is that sex effects (like hormonal changes) have to be seen in the larger context of the many other neurochemical processes going on in the brain, and that “it takes a lot of neurons to mobilize a mood.”42

  This point about looking at the bigger picture brings us to a critically important possibility. What if the purpose of some sex differences in the brain were to counteract other differences? The numbers 3 and 2 are different from the numbers 4 and 1, but both combinations achieve the same additive result. Likewise, as University of Massachusetts Boston psychobiologist Celia Moore points out, different brains can reach the same ends via different neural means.43 This is where evolutionary-flavoured preconceptions become key. For instance, in a Psychology Today blog subtitled “New Study Confirms That Men’s Minds Come from Mars and Women’s from Venus,” University of Chicago psychobiologist Dario Maestripieri writes that, “from an evolutionary perspective, large differences in personality between the sexes make perfect sense.”44 From this conceptual starting place, what could make more sense than to suppose that any particular sex difference in the brain (or hormones) serves the purpose of making the sexes behave differently? It is easy to overlook the point that, to the extent that males and females need to be able to potentially behave in similar ways to get by in day-to-day life, evolution has to work out a way for this to be achieved in the somewhat different bodies with which they’re bestowed. Humans, it’s worth pointing out, rank pretty low on the Spectacular Bodily Sex Differences Scale. As Occidental College sociologist Lisa Wade points out, “If we were as sexually dimorphic as the elephant seal, the average human male would tower six feet above the average woman and weigh 550 pounds.”45 By contrast, beyond the genitals, women’s and men’s bodies overlap in everything from hormones to height, but there are average physiological differences. And so, bearing in mind that “male neural systems have evolved to control behavior most optimally in a male body and likewise for females,” as de Vries and a colleague suggest, we can’t assume that neurobiological sex differences always act to create differences in behaviour. Sometimes, they may in fact serve to iron them out.46

  A beautiful example of de Vries’s principle of “compensation, comp
ensation, compensation”47 comes from the neuroscience of bird-song, recently explained by Fausto-Sterling, in her book Sex/Gender: Biology in a Social World.48 Songbirds represent one of the few success stories in linking sex differences in the brain to sex differences in behaviour. In the canary, for instance, the “song-control” brain region is bigger and denser in males, and this has been directly linked to males’ superior singing. There’s likewise a hefty sex difference in that brain region in another songbird species, the African forest weaver bird, being one and a half times larger in males. But unexpectedly, in this species, males and females sing together, in unison. How do both sexes sing the same song, despite this large sex difference in the song-control region of the brain? The answer: because of another sex difference. In females, the genes involved in song production areas “express” (produce brain-altering proteins) at a much higher rate than in males, compensating for their smaller neural real estate. “In effect,” Fausto-Sterling explains, “the gene action advantage canceled out the size advantage leading to equal song production abilities.”49

  But still. These caveats and principles are all very well and good, but rats and other animals nonetheless don’t have perfectly gender-egalitarian lifestyles. Yes, sexual differentiation of the brain is proving to be messier, more complex, and variable than previously appreciated. It’s even less, well, sex-y than once thought, in the sense of sex being one of many interactive factors, rather than acting as a single, clear, predictable director of development. Yet somehow all of this still gives rise to certain kinds of behaviour that are more common in males, and other kinds of behaviour more common in females.50

  This is a fair point. But easily passed over as a stabilizing buffer that allows the emergence of sex differences in behaviour is the environment.51 Decades ago, Moore found that the high levels of testosterone in the urine of male newborn rats triggers a higher intensity of anogenital licking by their mothers, compared with the amount of licking received by female pups. This extra licking turns out, she found, to stimulate the development of sex differences in brain regions that underlie basic mating behaviour.52 More recently, this more devoted maternal licking of males has also been linked to epigenetic effects in the brain and sex differences in youthful play behaviour, potentially a precursor to later sex roles.53 In other words, the mothers’ behaviour is an integral part of how male rats’ brains and behaviour develop differently from females’.

  This seems remarkable. Maternal care, a critical part of evolution’s strategy for creating something as fundamental as male sexual behaviour? Shouldn’t something so elemental be in the portfolio of the genes? But as developmental biologists have been pointing out for decades, offspring don’t just inherit genes. They also inherit an entire “developmental system”: an ecological legacy of place, physical environment, and structures; and a social legacy of parents, relatives, peers, and others who also provide important and reliable inputs as the animal grows and learns.54 A rat will be born to a mother that will lick its anogenital region. A primate will be born in an environment with ready access to fruit. In other words, genetic material isn’t the only source of developmental building blocks that can be relied on to be stable and enduring. So why not exploit this? Just as car engineers don’t bother to design miniature crude oil distillers into cars since gas stations are readily available to motorists, “selection cannot favour a trait that compensates for the loss of a developmental input that is, as a matter of fact, reliably available,” as University of Sydney philosopher of science Paul Griffiths explains. For example, primates have lost the capacity to synthesize vitamin C; why bother retaining this ability, since vitamin C is readily available on fruit trees?55 Similarly, if a mother rat that enthusiastically licks one’s anus and bottom is something that a male pup reliably inherits, along with his DNA, then natural selection will make use of it.

  We humans obviously don’t have anogenital licking as a means to provide a different developmental system for males and females. But the list of what we do have—a.k.a. gender socialization—is seemingly endless. No sooner has the determined gender scholar decided that she has completed her inventory, when Bic releases a special, slim pen “for her,” or Oster creates an “Ironman” blender for males’ very specific and distinct food-blending needs.56 In newborns, small average sex differences in size, health, and ability to self-regulate might influence caregiver-baby interactions, even before parents’ gender-related beliefs fully kick in, suggests Fausto-Sterling.57 But it’s the genitalia—and the gender socialization this kicks off—that provide the most obvious indirect developmental system route by which biological sex affects human brains. De Vries and Forger suggest that one way to think about these kinds of indirect pathways is that, ultimately, sex recruits a range of resources to reproductive-related ends. In other words, it delegates some of its developmental work to external contractors. And (as any home renovator knows), the bigger, longer, and more complicated a project is, the more scope there is for the end result to depart from the initial vision. And so, de Vries and Forger suggest, when it comes to humans, “with extensive social interactions and long development times, this means that there are plenty of opportunities to override or, alternatively, magnify the initial ‘program.’”58

  This is certainly one respectable view of gender socialization. Yes, we press dolls almost exclusively on girls; yes, we have a sexual double standard, and so on, and these social factors do make a difference. However, according to this view, these social norms exist because they reflect and respond to the original “programme” with which sex endows us: “nature” recruits “nurture.”59 But is there a particular “programme” or outcome that males and females are supposed to “develop to”?60

  Some neuroscientists speculate that a benefit of environmental influences (like maternal behaviour) having a hand in sexual differentiation of the brain is that the process can therefore be tinkered with, in a way that’s helpful for whatever the current environmental conditions happen to be.61 And in our own species, this capacity is beyond useful: it’s essential. The diversity of environments—and therefore conditions and roles—to which we need to have the potential to adapt far surpasses that of any other creature. Consider the variety of ways we feed ourselves, even: “It seems certain that the same basic genetic endowment produces arctic foraging, tropical horticulture, and desert pastoralism, a constellation that represents a greater range of subsistence behavior than the rest of the Primate Order combined,” two evolutionary scientists note.62 This critical difference between ourselves and other species is perhaps best illustrated by the reality TV show Wife Swap. In this long-running programme, viewers enjoy the mayhem that ensues when wives of generally very different social class, background, personality, and lifestyle swap homes, household rules, lives, husband, and children for two weeks “to discover just what it’s like to live another woman’s life.”63 I think I can say with low risk of the charge of anthropocentric bias that there is no other species in the animal kingdom for which this concept would work for seven seasons. Other animals are fascinating, to be sure. Many are highly flexible and adaptable. But there just aren’t that many ways to be a female baboon. The unrivalled interest of human beings as objects of examination for reality TV programming reflects the fact that, as evolutionary biologist Mark Pagel puts it, we are “a single species with a global reach and ways of life as varied as collections of different biological species.”64 The anthropological, historical, psychological records and a single episode of Downton Abbey clearly show that how women and men behave “varies greatly depending on situations, cultures, and historical periods,”65 as psychologists Wood and Eagly put it. We saw earlier in the book that even when it comes to something as basic as bringing the next generation into being, we humans have enjoyed an array of possibilities as to how to get the job done. A man might be a Chinese emperor with a large harem to service, or a contentedly monogamous British civil servant. A woman might be a mail-order bride, or actively seek mult
iple lovers in a socially sanctioned arrangement.66 (Not even sexual preferences, despite being pretty important when it comes to the possibility of reproductive success, are reliably and exclusively other-sex oriented across people, times, and contexts.) It’s simply not possible to designate any one way of life as representative of “male sexuality” or “female sexuality.” So, too, for parental care: although greater maternal care seems to be universal across time and place, both mothers and fathers can be negligent and abusive, or loving and attentive, while cultural norms span from wet nurses to breast-feeding on demand, from boarding schools and thrashings to permissive, helicopter parenting. And as Wood and Eagly document, although it’s universal for human societies to have a division of labour by sex, how those roles are shared, and what they involve, vary markedly across time, place, and circumstance, depending on the demands of the “cultural, socioeconomic, and ecological environment.”67 These very open-ended outcomes would be more easily achieved by a developmental pathway that runs from sex to socialization to the brain (and hormones, as we’ll come to in a later chapter), rather than by an inflexible direct path from sex to brain.68

  True, human societies’ allocation of sex roles isn’t always arbitrary: some roles are universally performed more commonly by one sex or the other. These, suggest Wood and Eagly, track physical differences between the sexes: in particular, men’s greater upper body strength versus women’s unique ability to grow and, until the invention of infant formula, feed babies. These make jobs like hand-to-hand combat and chopping wood more suited to men’s physiques and, historically, jobs that required stretches of time uninterrupted by hungry babies would have tended to be allocated to men. But even these consistent sex role divisions aren’t absolute. Sometimes, as Wood and Eagly describe, ecology and circumstances align in ways that bring about highly counter-stereotypical roles. For example, in some hunter-gatherer societies fathers show intensive care of infants, while in others women hunt large game, or hunt with dogs and nets,69 or take part in military combat, including very occasionally in all-female units.70