Men and women showed significant differences in their response to negative images in a total of 17 regions. The intriguing pattern was that women showed significantly greater activation than males in all of the major regions of the limbic system: the left amygdala, hippocampus, thalamus, hypothalamus, and the medial frontal and anterior cingulate gyri. The only region among the major components of the limbic system with a disproportionately high male activation was the left putamen, a part of the basal ganglia that is not known to have any relationship with emotion (it has many functions, but mostly involving control of motor skills). To put it another way, when it came to negative emotion, females had significantly stronger responses in those parts of the brain that play the most important part in generating emotion; males had significantly stronger responses in regions of the brain that are only peripherally involved in generating emotion.
Now consider what happens when men and women are exposed to positive images. Men showed significantly greater activation than women in the left amygdala, but, with that single exception, neither sex had significantly greater activation in any component, major or minor, of the limbic system traditionally defined. All of the other areas in which there were significant sex differences were elsewhere in the brain. Thus a first and potentially important sex difference: Women have a pronounced neurological tendency to respond to negative stimuli; men have a pronounced neurological tendency to respond to positive stimuli.
The Stevens meta-analysis is useful for establishing the reality of an overall relationship—in this case between regional sex differences in the brain and emotional response—but the best individual studies tell us more about what’s going on.
Rumination. In the psychological literature, rumination refers to thoughts, typically autobiographical, that a person mentally rehearses over and over, usually not productively. When they are negative thoughts, rumination amounts to brooding. Taken to an extreme, rumination can become depression. In the 1990s, Susan Nolen-Hoeksema led several studies establishing that women were more likely than men to ruminate, particularly in response to negative events.124 In the early 2000s, two studies using brain imaging established a biological basis for those findings. Instead of showing participants pictures just once, participants saw them several times. The finding in both studies was that males quickly became habituated to a stimulus—the response in the amygdala decreased rapidly after the first few exposures—whereas it persisted among women.125 In 2013, researchers at the Harvard Medical School (Joseph Andreano was the first author) tested whether this pattern replicated for both positive and negative stimuli. It did not. As in previous studies, men showed higher amygdala activity for novel stimuli than women no matter whether the stimulus was negative, neutral, or positive. For familiar positive stimuli, men again had a higher response than women. But when it came to negative stimuli, men quickly habituated while women continued to show substantial amygdala activity even after repeated exposure. The difference was large enough that it reached statistical significance despite the small sample size.126
Now recall the table in chapter 2 that showed the prevalence of personality disorders by sex. Men had higher incidence rates on the autism spectrum, conduct disorders, ADHD, and schizophrenia, among others. Women had higher prevalence on another set, including three that involve rumination: major depression, generalized anxiety, and post-traumatic stress disorder. The findings I have just summarized point to a sex difference both in the intensity of initial reaction to negative stimuli and in the persistence of that reaction, which in turn point to a difference in rumination. In discussing the meaning of its findings, the Andreano study put them into the context of the literature on depression and the brain:
Persons at increased risk for both depression and anxiety disorders exhibit amygdala activity in response to both novel and familiar faces, while controls respond only to novel faces.127
Persons at increased risk for those disorders continue to show increased amygdala response to faces even after extended habituation.128
Persons high in trait anxiety fail to habituate to previously threat-associated stimuli, as these stimuli continue to evoke amygdala activity after extinction training.129
Persons with post-traumatic stress disorder show decreased habituation in amygdala signal to repeated fearful faces.130
Individuals with more persistent amygdala response to negativity in the Andreano sample also self-reported more anxiety and depressive symptoms than those with faster habituation.131
Spontaneous intrusive memories and mood-congruent recall have been related to increased amygdala activity.132
Increased engagement of the amygdala during the encoding of emotions predicts subsequent memory.133
Putting together the pieces, Andreano and his coauthors concluded that “a resistance to habituation to negative material in women may represent a potential vulnerability contributing to women’s higher rate of affective disorder.”134
I have given you a typical example of how progress is being made in linking phenotypic sex differences—in this case, a conspicuous sex difference in depression and related disorders—and their biological underpinnings. It comprises an intricate map of related phenomena that have been observed and implications that have been substantiated.
At the end of it, we still are looking at an incomplete picture. But this example is also typical in the velocity of discovery that it represents. At the turn of the twenty-first century, it was known that the incidence of depression was higher among women, that women ruminate more than men, and that there was probably some relationship between those facts. Two decades later, important components of the biological processes of depression are understood and progress continues to be rapid. When the full etiology of depression is known, it may well be that environmental influences explain some of the sex differences in prevalence of depression. But even now, convincing evidence indicates that biology is also part of the story.
Recapitulation
The takeaways from this chapter’s complicated discussion can be summarized quickly:
Circulating sex hormones produce easily observable differences in the phenotype. Those hormones have specific, documented effects that match up with some of the differences in personality and neurocognitive functioning discussed in chapters 2 and 3.
The underreported news about sex hormones is the permanent effect that prenatal and infant surges of testosterone have on masculinizing the male brain. Those effects also match up with the earlier discussions of personality and neurocognitive functioning.
The greater lateralization of the male brain has been documented by a variety of evidence about sex differences in structural connectivity and functional connectivity. These findings bear on phenotypic sex differences in visuospatial and verbal skills.
Differences in the functioning of the amygdala, hypothalamus, and other regions of the limbic system appear to have links with phenotypic sex differences in memory and vulnerability to depression.
These topics barely scratch the surface. For example, I described sex differences in memory as they are related to the amygdala. Researchers are now integrating the findings on the phenotypic sex differences in memory, spatial abilities, and perceptual processing (the temporal order in which a scene and its individual features are recognized) into an explanation that invokes sex differences in the amygdala, hippocampus, and lateralization, all mediated by the locus coeruleus in the brain stem and the adrenal glands (more precisely, the catecholamine system).135 For those of you who want to get a broader sense of how much has been done and is under way in research on sex differences in the brain and are prepared to cope with some densely technical material, I recommend a 2019 review article, “Sex Differences in the Developing Brain.”136
Probably the takeaway with the most long-term importance is that it’s still early days. The progress in understanding sex differences in the brain over the last two decades has been spectacular, but you can expect it to be eclipsed by what will be learne
d in the next twenty years.
A Personal Interpretation of the Material in Part I
I reserve an entire chapter at the end of the book for my own interpretation of larger issues, but I also end Parts I, II, and III with personal statements of my reading of the material.
Males and females are different. A lot different. The distinctions that show up in the phenotypic evidence on personality, abilities, educational choices, vocational choices, and career paths are interconnected both conceptually and empirically. The links between these phenotypic differences and the sex differences in the brain are still only partly understood, but what we have learned so far also hangs together. I expect that the more we learn, the more closely phenotypic differences will match up with genetic differences. I will also assert without trying to demonstrate it that this coherent picture fits seamlessly within the context of evolutionary pressures over millions of years that shaped Homo sapiens.
But this is also a good time to remind you that “a lot different” does not come close to comprehensively different. On the contrary, those who would try to make the case that one sex is superior to another should recall some of the personality traits described in chapter 2 on which males and females do not appear to differ. Some of those involve personality traits that many men like to associate with being male, such as forcefulness in expression, self-reliance, and venturesomeness; others involve traits that many women like to associate with being female, such as openness to the inner world of the imagination, spontaneity, and openness to new experiences. In those instances and many other important traits such as commitment to fulfilling moral obligations and thinking things through before acting, males and females are indistinguishable.
As the discussion of abilities in chapter 3 should have made clear, males and females do indeed differ in their profiles of abilities—but in such complicated ways that claiming superiority for one sex or the other is ridiculous. Or I’ll put it another way: Claiming superiority can be done only by attaching subjective weights to different strengths. Revealing what those weights are exposes how subjective the claims are.
As for the differences in educational and vocational choices discussed in chapter 4, decisions about what makes for a satisfying vocational life are intimately bound up with personal preferences and priorities. Inborn sex differences in personality and abilities contribute to different distributions of vocational preferences and priorities. Sex differences in this domain will be with us forever. They are not to be deplored but celebrated.
PART II
“RACE IS A SOCIAL CONSTRUCT”
Peoples of the world have probably had words that mean “people different from us” as long as they have had language. A common practice in isolated tribes has been to call one’s own tribe humans and everyone else nonhumans. By the end of the sixteenth century, the word race had entered the English language, originally used loosely to refer to people of common descent, identified with their common culture and geographic place. Increasing contact with the peoples of Africa and Asia led to distinctions based on differences in appearance. In popular usage, whites in Europe began to group races based on skin color—white, black, yellow, brown, and red.
In the eighteenth century, science got involved. Naturalists Carl Linnaeus and Johann Blumenbach proposed formal groupings of populations into races based on distinctive morphological features. By the middle of the nineteenth century, scholars had decided that the different races were not only cosmetically and morphologically distinctive but also had different personality and intellectual characteristics. The differences amounted to a racial hierarchy, they argued, with whites on top and blacks at the bottom.
These scientific writings occurred in the context of the Europeans’ colonization of the New World. In South and North America alike, the intruders displaced and in many cases eradicated the indigenous peoples who already occupied the land. They enslaved and imported African blacks and incorporated slavery into their social systems.
The consequences were devastating. In the opening to Part I, I described the legal status of English women through the eighteenth century as not much short of de facto slavery. The effects of actual slavery experienced by Africans in the New World went far beyond legal constraints, and they were far worse on every dimension of life. The freedom granted by emancipation in America was only marginally better in practice and the situation improved only slowly through the first half of the twentieth century. Meanwhile, all of the indigenous cultures of the New World had been devastated beyond recognition by the end of the nineteenth century. For the United States, founded on ideals of liberty and equality, that record was a fatal flaw that in my view ensured the eventual unraveling of the American project.[1]
Among scholars, the opening of the twentieth century saw a scientific backlash not only against the idea of a racial hierarchy but against the idea of race itself. Its most prominent spokesman was Franz Boas, a pioneering anthropologist and a fierce opponent of what he labeled “scientific racism.”2 A British anthropologist who studied under Boas, Ashley Montagu, took his mentor’s position to new levels of passion (“Race is the witchcraft, the demonology of our time”) and set the rhetorical tone for today’s academic orthodoxy. The book from which that quote is taken, Man’s Most Dangerous Myth: The Fallacy of Race, was originally published in 1942 and remained in print throughout the rest of the century.3
In the 1970s and 1980s, the backlash against the concept of race got new ammunition with two propositions: The genetic differences among human populations are insignificant, and humans left Africa too recently for important differences to have evolved. These arguments were most famously expressed by geneticist Richard Lewontin and paleontologist Stephen Jay Gould, both of Harvard.
In 1972, Lewontin published an article titled “The Apportionment of Human Diversity.” In it, he analyzed genetic diversity among the different races with the tools available at the time and found that less than 15 percent of all genetic diversity is accounted for by differences among groups. He concluded with a passage that has since become canonical:
It is clear that our perception of relatively large differences between human races and subgroups, as compared to the variation within these groups, is indeed a biased perception and that, based on randomly chosen genetic differences, human races and populations are remarkably similar to each other, with the largest part by far of human variation being accounted for by the differences between individuals.
Human racial classification is of no social value and is positively destructive of social and human relations. Since such racial classification is now seen to be of virtually no genetic or taxonomic significance either, no justification can be offered for its continuance.[4]
The canonical version of the orthodoxy’s second proposition appeared twelve years later, written by Gould for his regular column in Natural History magazine. “Equality [of the races] is not given a priori,” he wrote.
It is neither an ethical principle (though equal treatment may be) nor a statement about norms of social action. It just worked out that way. A hundred different and plausible scenarios for human history would have yielded other results (and moral dilemmas of enormous magnitude). They didn’t happen.
Gould argued for this conclusion along several lines, some of which echoed Lewontin. But he also offered a new proposition that quickly became popular: “[T]he division of humans into modern ‘racial’ groups is a product of our recent history. It does not predate the origin of our own species, Homo sapiens, and probably occurred during the last few tens (or at most hundreds) of thousands of years.” For Gould, the implication was obvious:
As long as most scientists accepted the ancient division of races, they expected important genetic differences. But the recent origin of races… squares well with the minor genetic differences now measured. Human groups do vary strikingly in a few highly visible characters (skin color, hair form)—and this may fool us into thinking that overall differences must be great. But we now know
that our usual metaphor of superficiality—skin deep—is literally accurate.
And so, he concluded in his 1984 article, “Say it five times before breakfast tomorrow; more important, understand it as the center of a network of implication: ‘Human equality is a contingent fact of history.’”5 Gould stuck to that position for the rest of his life. In an interview in 2000, he made the blanket statement that “natural selection has almost become irrelevant in human evolution. There’s been no biological change in humans in 40,000 or 50,000 years. Everything we call culture and civilization we’ve built with the same body and brain.”6
The implication was obvious: The concept of race has been made up—or, put more academically, socially constructed. Sociologists Michael Omi and Howard Winant supplied a theoretical framework for the social construction of race in 1986 with the publication of Racial Formation in the United States: From the 1960s to the 1980s. Racial formation, they wrote, refers “to the process by which social, economic, and political forces determine the content and importance of racial categories.”7 Race, they went on to argue, is an artificial way of assigning people to groups, consolidating the power of the majority that sets the rules for racial assignment and enabling that majority to control racial minorities.
The orthodox sometimes come surprisingly close (given the obvious cosmetic differences across races) to asserting that biological race is a figment of our imaginations. I’m not talking about people at the fringes of academia. An official statement of the American Sociological Association in 2003 told its members to beware “the danger of contributing to the popular conception of race as biological.”8 Nor am I talking about attitudes that have softened in the face of all that has been learned since the sequencing of the genome. Here is part of the official statement of the American Association of Physical Anthropology on race that was adopted on March 27, 2019:
Human Diversity Page 16