MARC D. HAUSER
Psychologist, Harvard University; Author, Wild Minds.

It is hard to imagine anyone living today disagreeing with Baron-Cohen's starting premise that there are biological differences between the sexes. Even the staunchest cultural relativists have to acknowledge that there are differences in the sex chromosomes and hormonal titers that lead directly to differences in our anatomy.

Recent work on imprinted genes a class that fails to follow the classic Mendelian patterns of inheritance shows that maternal contributions are often in complete conflict with paternal contributions. For example, with some imprinted genes, the maternal copy is quiet and the paternal copy is expressed, causing the fetus to extract more from its mother than she would like; these genes often cause pregnancy complications such as gestational diabetes. Studies of the brain using neuroimaging reveal sex differences in structure and function, and work with patient populations reveal differences in vulnerability to mental disorder. And closer to home, there are massive sex differences in the incidence of autism, with studies reporting an 8:1 bias in favor of males.

Where the debate gets interesting is when one attempts to explain how tightly the biology constrains our thoughts, preferences and actions. Baron-Cohen's assortative mating hypothesis is an attempt to grapple with this issue. Much of the evidence hinges on the early appearance of sex-specific signatures of mental function. Early signatures are a tell-tale sign of an innate capacity peaking through, but they are not definitive. One needs to rule out that the experience obtained is insufficient for a learning mechanism to create the capacity.

And here is where Baron-Cohen's observation that newborn boys like to look at mobiles and little girls at faces is fantastic, and just the right kind of start into a serious research program on the biology of sex differences; these results fit nicely with other data showing that for spontaneously generated paintings by young children, little girls almost always draw one or more people into the scene, whereas little boys rarely do, using their canvas as a vehicle for vehicles, from rocket ships to more mundane cars and bicycles.

But now comes the hard work.

What is it about the male genome that sets up a preference for the mechanical or physical whereas the girl genome leans toward faces and the social? How quickly, and with what kind of experience, can these initial biases be exaggerated? Why did these differences evolve? In the language of Darwin, what selected for this kind of preference? Was it our division of labor, with males focused on hunting and therefore technology, while females focused on gathering and the schmoozing that goes on during this kind of activity?

One clue that these are evolved sex differences comes from recent work looking at the incidence of innovation among primates. Across all the primates, including our closest relatives the chimpanzees, males are far more likely than females to take the lead in innovation, and much of the creativity lands in the domain of tool technology. In contrast, for most primate societies, females are for more engaged in the intricacies of social life than are males, largely because females tend to stay in their natal groups for life whereas males emigrate out. If there is a bias toward male folk physics and female folk psychology, there may be traces way back to our primate ancestors.

How are data like Baron-Cohen's reconciled with the fact that for imprinted genes, maternally active copies appear to be largely expressed in the rational frontal lobes whereas the paternally active copies appear to be largely expressed in the emotional limbic lobes? Are there in utero battles that arise over the concentration of testosterone circulating during development, with paternally active genes pushing hard for increased testosterone to push growth and toughness? Are maternal copies pushing in a different direction, attempting to regulate the physiology in such a way that their offspring are social specialists?

What makes work like this so very difficult, especially in terms of selling it to the public, is that more often than we would like to admit, reported sex differences either crumble in the face of follow up work, or for those differences that have been reported and replicated, claims regarding biological underpinnings have fallen prey to more experientially-based accounts. One only need think back to gay genes and gay brains, and the sad fate of those results. Thus, although I am sympathetic to Baron-Cohen's research project and find it odd that anyone would consider this work controversial, there is an obligation to get the story right here that far exceeds the demands in other areas.

STEVEN PINKER
Psychologist, Harvard University; Author, The Blank Slate.

I find Simon Baron-Cohen's work admirable in several ways. The systematizer-empathizer dimension is an interesting new way to capture some of the variation between male-typical and female-typical cognitive styles. It cuts across motivation and aptitude (which are often difficult to distinguish) and might subsume some of the long-noted sex differences that have been stated in cruder form, such as orientation to objects versus people. I also am intrigued by the studies of the effects of fetal testosterone, a valuable new way to learn about which psychological sex differences might be consequences of the biological programs that build our brains.

Baron-Cohen wonders why sex is so often referred to these days as "gender." Part of it is a new prissiness — many people today are as squeamish about sexual dimorphism as the Victorians were about sex. But part of it is a limitation of the English language. The word "sex" refers ambiguously to copulation and to sexual dimorphism, and it's often important not to confuse them! The linguistic term "gender" literally means "kind," as in the cognates "genus," "generic," and "genre." Languages often subdivide their nouns into kinds for purposes of inflection, such as human/nonhuman, animate/inanimate, long/flat/round, vowel-final/consonant-final, and male/female. Many Indo-European languages have a gender distinction in their nouns that aligns with a masculine/feminine distinction in their pronouns, and so "gender" was pressed into service as a way to refer to the difference between men and women. Some academics want "gender" to refer specifically to socially defined rather than biologically determined patterns of sex-typical behavior, but this guideline, like most top-down prescriptions about lexical semantics, is rarely obeyed. The basic problem is that we have three concepts to convey — intercourse, dimorphism, and social roles — and at best two words with which to convey them.

I was amused to read that "It may be simply that the climate has now changed, and that people are much willing to accept that there are sex differences in the mind, and that these might even be partly biological." Was this interview conducted before the event that is coming to be known as "1/14"?

[ED. NOTE: The interview took place at Trinity College on 3/12.]

ARMAND LEROI
Biologist, Imperial College; Author, Mutants.

I take the premise of Simon Baron-Cohen's project — that there are innate sexual differences in behaviour and aptitudes — for granted. As Olivia Judson recently pointed out in a New York Times piece about "1/14", it "s hard for a zoologist to suppose otherwise.

I am not, however, wholly convinced by his argument that autistic children — nearly always boys — are, in effect, hypermales. Baron-Cohen has shown that, relative to girls, boys are good at systematising and poor at empathising, and that autistic boys are exceptionally so. This fascinating result then raises a question, namely, why should these two, seemingly unrelated, attributes should trade-off with each other? 

Baron-Cohen's answer seems to be: foetal testosterone. Perhaps autistic boys were exposed to unusually high levels of testosterone in the womb, so developing the systematising part of their brains, and repressing the empathising part. It's an exaggeration of a normal process. This strikes me as perfectly plausible, but it also entails a number of peculiar, if testable, consequences.

Testosterone is a hormone and, as such, affects the entire brain and body of the foetus. This means that one should expect autistic boys to be hypermales not just with respect to systematising and empathising — but in all possible ways. 

Are autistic boys, then, exceptionally aggressive at play? When they grow up, are they invariably heterosexual? Do they look exceptionally masculine — that is, are they morphometrically hypermale? And what of those rare autistic girls? If they, too, are hypermale due to heavy doses of foetal testosterone, should this not be reflected in their behaviours, their sexual orientations, their bodies?

The motivation for these questions is that we do know something about the consequences of high foetal testosterone — witness the Spotted hyena. In all mammals, the placenta produces a lot of testosterone. This testosterone is, however, broken down into oestrogen by an enzyme called Aromatase. Spotted hyenas foetuses have naturally low levels of Aromatase, and so are exposed to extraordinarily high levels of placental testosterone. The result of this is that females are born with large penis-like clitorises that they can jaunt erect in dominance displays, fused vaginas and very nasty tempers.

Now, clearly we can't ask Spotted hyenas to systematise and empathise. But we can ask their human equivalents. Loss-of-function Aromatase mutations are occasionally found in humans. Like the hyenas, girls homozygous for such mutations are born with masculinised genitalia. Nothing, I think, is known about their temperaments or talents. Are they autistic? Better yet, are their brothers homozygous for the same mutations, autistic as well? If Baron-Cohen is right, they should be.

Of course, even if the hypermale theory of autism is wrong, Baron-Cohen's proposed mechanism of assortative mating among systematisers could still be right. But let us hope not. Surveying my colleagues it seems to me that the reproductive success of gifted female scientists is poor enough as it is.

CAROLE HOOVEN
Lecturer in Anthropology, Harvard University.

Simon Baron-Cohen's Assortative Mating Theory of Autism is ambitious, and demonstrates the usefulness of innovative, "big picture" thinking in getting a handle on seemingly intractable problems. The ideas seem to fit together nicely to present a clear picture of the etiology of autism. The flip-side of big-picture thinking, however, is that the authenticity of the full picture relies on the validity of each sub-theory; and in this case, the big-picture is painted with a series of appealing, yet tentative strokes. Baron-Cohen acknowledges that the underlying theories need to be tested, and he presents five sub-theories, in the form of testable hypotheses, which focus primarily on the relationship between genetics and systemizing.

His main idea is as follows: people process the world using a cognitive style that falls somewhere along the spectrum of systemizing (more masculine) to empathizing (more feminine). In systemizing, information is processed with attention to rules and laws, and in empathizing, processing is biased toward social cues. As autism can be characterized by the combination of two extremes — a lack of attention to social cues and a focus on laws and rules — then it follows that, should there be genes for systematizing, two systemizers have a higher chance of producing an autistic child. It further follows that, should systemizing be a predominantly masculine trait, the agent that masculinizes the brain, prenatal testosterone, may be also play a central role in both systemizing and autism.

A wealth of evidence, including Baron-Cohen's own research, links the effects of early testosterone to later masculine behavior in humans and non-human animals. Some of the most robust research has examined the effects of Congenital Adrenal Hyperplasia (CAH), in which too much testosterone is produced by the adrenal gland prenatally. Females with this condition (whose hormonal levels are normalized at birth) are somewhat masculinized—e.g., they engage in relatively high levels of rough and tumble play and perform in the male-typical range on spatial tests (higher than normal females). If early testosterone increases systematic processing and masculine behavior, then it follows that CAH boys should also show increased performance on tests of spatial ability; but among males, the relationship between perinatal testosterone and later spatial performance is equivocal at best. Indeed, there is evidence to suggest that excess testosterone in utero actually hampers male spatial ability.

A big-picture, evolutionary analysis of the Assortative Mating theory reveals somewhat of a paradox between conventional notions of masculinity, and the newer notions of "cognitive masculinity." Testosterone can be thought of as promoting behaviors that are traditionally masculine, preparing males physically and psychologically to bias energetic investment toward mating effort. In adult males, high testosterone levels are associated with status-seeking behaviors and the pursuit of mating opportunities. In men, confidence and social dominance (which would require a relatively high social facility) are predicted by high testosterone. The case of the classic nerdy scientist conjures up images of the stereotype of the low testosterone, but in the current context "cognitively masculine," man — a scrawny male who, although he may be successful in the world of technology, is a miserable failure socially and romantically.

The paradox of the two notions of masculinity raises questions about the role of testosterone in shaping psychological traits, such as status-seeking behavior and spatial ability, in utero and in adulthood. With my colleagues Chris Chabris, Peter Ellison and Steve Kosslyn, I've investigated the role of testosterone in solving spatial problems. We have found that although high testosterone males outperformed low-testosterone males on mental rotation tests, the high performers gained their advantage not because they were better at internally transforming objects, but, as the evidence suggested, because they were more confident in their decisions about the similarity of objects. Perhaps the paradox can be at least partially resolved by furthering our understanding of testosterone's role in affecting performance on cognitive tests.

These findings on individual differences in mental rotation performance, along with a relative lack of robust findings on the effects of perinatal testosterone, remind us that picture of how testosterone affects cognition is still far from complete. While evidence strongly suggests that early and late sex differences in testosterone levels play a central role in shaping traits that are clearly related to mating effort, such as dominance, sexual, and competitive behaviors, we should not take for granted that testosterone affects cognitive ability directly. More research should examine the extent to which the relationship between testosterone and cognitive performance is actually modulated by a third variable, having to do with dispositional factors such as confidence and competitiveness, that are more closely associated with mating effort.

ELIZABETH SPELKE
Psychologist, Harvard University.

We humans seem to have an abiding need to reduce the richness, variety, and complexity of our mental lives to categories. In past times, we divided ourselves into the introverts and the extroverts, the field-dependent and the field-independent, the visualizers and the verbalizers. With the advent of neuroscience came the chance to divide our brains into categories as well. That supremely intricate and elusive organ became the left brain and the right brain, the grey matter and the white matter, the male brain and the female brain. Simon Baron-Cohen builds on the last distinction and offers us a new pair of categories by which to sort ourselves: we are systematizers or empathizers.

The categories of the past have a quaint look about them, because none of them has proved very satisfying. Binary categories don't buy us much, because humans are both more and less variable than they suggest. Two categories are far too few to pigeonhole a species whose members can grow from newborn infants to seal-hunters, cathedral builders, or capoeira dancers. Yet two categories are also one too many. In the right circumstances, all of us become introverted or extroverted, swayed by others or steadfast to our principles, visually imaginative or verbose. Global categories tend to obscure the commonalities in human experience and the common capacities, hopes, and failings that define us as a species.

Is there a male brain, and is systematizing its job description? Baron-Cohen asks us to distinguish politics from science and consider the evidence. The evidence for an inborn, male predisposition for systematizing comes from a single experiment on newborn infants, tested with a single person and object. The person was the report's first author, who surely knew the experimental hypotheses and who, we now learn, may have known the sex of the infants whose attention she elicited. The experiment provides no evidence that the basis of infants' preference, if real, had anything to do with the categorical distinction between the displays. Would infants show the same preferences for other face/object pairs? Would they maintain this preference if low-level properties of the two displays, such as their speed of motion, were equated? One need not object to Baron-Cohen's politics to be less than persuaded by his data.

More important, Baron-Cohen fails to consider the extensive evidence that has accumulated, over the last thirty years, on infants' developing understanding of object mechanics. Hundreds of well-controlled experiments reveal no male advantage for perceiving objects or learning about mechanical systems. In most studies, male and female infants are found to discover the same things at the same times. Both males and females come to see the complete shapes of partly hidden objects under the same conditions and at the same ages. They figure out how objects support one another, through the same series of steps. They reach for objects by extrapolating their motions, with equal accuracy. They make the same errors when they search for hidden objects, and they get over those errors at the same time. Sometimes female infants have an edge: In experiments by Laura Kotovsky and Renee Baillargeon, for example, females start to learn about the relation between force and acceleration (the harder a stationary object is hit, the further it goes) a month earlier than males do. Males catch up, however: by 6 1/2 months, you can't tell them apart.

Whatever the newborn infants in Baron-Cohen's experiment were doing, the male and female participants in three decades of infant research have followed a common path, engaging with objects and people. Infants don't choose whether to systematize or empathize; they do both, and so do we all. Baron-Cohen's categories may seem as quaint as left and right brains by the time his newborn subjects are old enough to read about them.

ALISON GOPNIK
Psychologist, UC-Berkeley; Coauthor, The Scientist in the Crib

A quick follow-up to Liz Spelke's point. In hundreds of studies of children's Theory of Mind there is no evidence of sex differences or a female advantage. These include hundreds of studies of the very false belief task that Baron-Cohen, in excellent earlier work, showed was strikingly deficient in people with autism, as well as many other studies of other similar abilities. There are other individual differences, younger siblings, for example, show a consistent advantage over older ones. But not sex differences.

HELENA CRONIN
Director, Darwin@LSE; Author, The Ant and the Peacock.

If Simon Baron-Cohen's theory is true, it must be compatible with the workings of natural selection ­ in particular with our Darwinian understanding of the evolution of sex differences in all species.

Baron-Cohen's view is that psychological sex differences in our species can be characterized as male minds tending more towards systemizing than empathizing, female minds vice versa.

The Darwinian understanding of sex differences is that they arise from a trade-off between two kinds of reproductive investment: competing for mates and caring for offspring. Systematic sex differences in investment have been the rule almost since the dawn of high-tech sexual reproduction about 800 million years ago. This is because sexual reproduction of that kind began with one sex specializing slightly more in competing and the other slightly more in caring; that divergence then rapidly became self-reinforcing; and so it widened over evolutionary time, with the differences proliferating and amplifying, down the generations, in every sexually reproducing species that has ever existed. Thus it is that, throughout the living world, males and females tend to differ in much the same characteristic ways. By and large, males are competitive, promiscuous, risk-taking, status-seeking, tenacious, flamboyant; females are less so, for all of the above.

How does Baron-Cohen's view of sexual dimorphism in our species fit within this larger framework? The task of integrating his perspective has not yet been tackled. But this unfinished business is clearly important; and the result could benefit Simon's thesis in several ways. Consider, for example, questions to do with adaptation.

The Darwinian view of sex differences is adaptationist; it explains the differences as resulting from the very different adaptive problems that males and females faced because of their very different reproductive strategies. This raises the question: Are systemizing and empathizing also adaptations? And, if they are, what adaptive problems did they solve and how did they solve them? Alternatively, if they are not adaptations, which adaptations are they the products of? And how accurately do the categories 'systemizing' and 'empathizing' capture a relevant division in these underlying adaptations?

Thus, if Baron-Cohen's categorization reflects adaptive categories — either because it pinpoints specific adaptations or because it maps accurately onto underlying ones — then it can be slotted into its proper place in our wider understanding of psychological sex differences in humans. And we could then assess the robustness and scope of his theory — how reliably and how comprehensively systemizing-versus-empathizing characterizes the entire spectrum of differences.

So, embedding the distinction between systemizing and empathizing within that wider understanding could transform what might otherwise appear to be a merely pragmatic, even arbitrary, distinction into a rich explanatory and predictive theory.

And that would open up exciting new vistas. So, for example, it could shed light on male-female differences in other species. Are there any other species whose ancestors also faced adaptive problems to which systemizing and empathizing are a solution? And if this sex difference is peculiar to our species, why?

For all these reasons and more, I look forward to seeing Baron-Cohen's intriguing insights integrated into our understanding of sexual dimorphism in general.

By the way, moving from the science to its uses... Discussing the claim that sex differences are "all environment and no biology", Baron-Cohen says that one can appreciate "that politically that was an important position in the 1960s, in an effort to try to change society". But no; one most certainly should not appreciate it. If you really want to change society, you need first to understand it. An a priori commitment to 'environment over biology' sounds more like ideological posturing than a serious attempt to solve society's problems.

LINDA S. GOTTFREDSON
Sociologist, School of Education, University of Delaware

Simon Baron-Cohen's work joins the search for causal mechanisms linking genes, brain, and behavior. The patterned variation by sex at all three levels of analysis provides clues to what those mechanisms might be (e.g., testosterone). Baron-Cohen employs those patterns to better understand, in particular, the etiology of autism and its much higher prevalence among males.

Variation is the raw material for much scientific analysis and for evolution itself, but public discussion of human variation seems mostly off-limits today. We are called upon to celebrate diversity but not notice difference; to observe a new etiquette that forbids utterance of supposedly tactless knowledge. Good feeling compels public ignorance. But bewildered or bemused, outraged or apprehensive, most scientists soldier on.

Baron-Cohen continues to investigate the nature of sex differences. His research on babies only 24 hours old, while needing replication, fits the larger pattern of sex differences in interests, personality, and abilities across the lifespan. For instance, at all ages and worldwide, females tend to be more interested in people and males in inanimate objects. As noted in earlier commentaries, humans are not the only primates showing this pattern. It would have been an exception to the rule had Baron-Cohen's team not found boys gazing more at the mechanical object and girls more at the human face. Like the habituation research now used to assay differences in cognitive ability among infants, his results provide prima facie evidence that socialization cannot be the sole cause of variation in social behavior. The interesting question is not whether meaningful innate sex differences exist, but how anyone could construe the preponderance of evidence otherwise.

Baron-Cohen argues that the distinction between "systematizers" (disproportionately male) and "empathizers" (disproportionately female) is especially important in the etiology of autism. He theorizes that genetic risk of autism rises when both parents are systematizers. While onto something important, his work might advance faster and persuade better if, rather than proposing a new distinction, it exploited existing evidence on the dimensionality and relatedness of human psychological traits (all of them heritable), particularly interests ("Holland's hexagon" of six modal types), personality (the "big five"—or three or seven), and abilities (the "3-stratum hierarchical model").

Researchers in vocational interest measurement, personality assessment, personnel testing, and differential psychology have spent a century parsing, cataloguing, and correlating these differences among individuals. They find a regular pattern of sex differences regardless of age, time, or place. It is not clear where Baron-Cohen's systematizer-empathizer distinction fits in this much-explored territory, but it would seem to map best onto dimensions in the non-cognitive realm: sympathetic vs. cold ("agreeableness" personality dimension), "realistic" vs. "social" vocational interests, or valuing "ideas" vs. "feeling."

Prevalence of autism has increased so much recently that some label it epidemic. Baron-Cohen must explain how this increase is consistent with evidence that autism has strong genetic roots. The two facts are not inconsistent, but many people assume they are. They fallaciously reason that if prevalence jumps within only decades (e.g., more violent crime, more women getting BAs in math), then the behavior in question must not be genetically influenced because the gene pool could not have changed during that time. But it need not have.

First, non-random mating can change the distribution of phenotypes in the next generation of the same gene pool. Baron-Cohen's theory would predict that more assortative mating for "systematizing" will lead to more autism in the offspring generation. It would operate like inbreeding, which increases the odds of offspring inheriting the same deleterious recessive allele from both parents. This explanation doesn't work well, however, for rates of socially important behaviors that fluctuate within a generation (e.g., criminal behavior, which has a heritable component), and perhaps not even for autism.

Second, environmental change matters, but not equally for all genotypes. When environments become more deleterious, the more susceptible genotypes are the first casualties. Environmental toxins may be one such factor in autism. Conversely, as environments become more favorable, some genotypes are better able to exploit the new opportunities. So, as barriers to women in education and work have fallen, the most talented and ambitious women have been the best placed to advance. In neither case has the gene pool changed—only the environments favoring some genotypes over others.

Third, different genotypes seek and evoke different experiences and environments. Outgoing, agreeable, feelings-oriented personalities prefer (and are preferred for) dealing with people; non-social, pragmatic, things-oriented personalities find a better fit working with mechanical objects and processes. When free to choose, the two types will gravitate toward different careers. They will also create different personal environments for themselves and their children. So, just as low-IQ parents don't create the most propitious environments for their genetically at-risk children, perhaps two systematizers provide non-optimal rearing for theirs too.