The dumb kids at Middlebury College had no idea what they were setting in motion when they stopped Charles Murray from speaking. At an instantly infamous 2017 lecture, students shouted down his speech, waited through a livestreamed discussion between him and a faculty member given from a private location, and swarmed him after the event, injuring the faculty member. Murray, you see, had been thinking about swimming back toward the fraught waters he and the late Richard Herrnstein had explored in 1994’s The Bell Curve — notions that traits such as intelligence are hugely important in determining who gets ahead in modern societies, and that gaps on those traits among social groups, including racial groups, could be partly genetic in origin. His wife had been telling him not to. “Confound it!” he recalls her saying after the Middlebury affair (“. . . or two syllables to that effect”). “If they’re going to do this kind of thing anyway, go ahead and write it.” And now, three years later, we have Human Diversity: The Biology of Gender, Race, and Class. This isn’t an intemperate screed meant to trigger oversensitive 19-year-olds, however. Instead, it’s a patient and generally cautious explanation of where the science stands in the three highly contentious areas mentioned in the subtitle: the biological underpinnings of sex differences, social-class differences, and racial differences. Those who’ve been following developments in these areas will find little that’s surprising. But those new to the topics will learn a lot, so long as they understand basic statistical concepts well enough to follow Murray’s often-a-bit-technical prose. Murray provides some of his own entry-level instruction, but it’s a little scattershot in terms of what concepts get an explanation in the text, which definitions are relegated to an appendix, and what terms the reader is simply expected to know. Murray begins with sex differences because they’re the most obvious and hard to deny, so I’ll do the same. Men and women have measurably different abilities, preferences, and behaviors; many of those differences do not seem to be shrinking in societies that strive for gender egalitarianism; and new research is establishing some connections between the sexes’ behaviors and their brains. There’s a long list of sex differences that researchers have found repeatedly, and they go well beyond physical size. Men are more likely to have autism, women depression. Women are more concerned with the well-being of others; men are more assertive. Men have stronger visuospatial skills, women better verbal ability. Men tend to have higher variation in ability; for example, the sexes have the same average IQ, but men are overrepresented among people with very high or very low IQs. Men and women also have markedly different interests, especially in that men are more likely to prefer working with things, women with people. These gaps are no doubt at least partly due to socialization and culture, but at least some almost certainly have a biological component. One way of seeing this is to look at what happens when societies adopt stronger norms in favor of gender equity: Do the gaps shrink, as would be expected if socially enforced gender norms caused the gaps to begin with? Sometimes, sometimes not. Murray goes through a long list of different trends. Male overrepresentation among high scorers on the math portion of the SAT, for example, has shrunk steadily for decades. Women’s movement into jobs that involve working with things rather than people, though, happened speedily in the 1970s and 1980s and stopped thereafter, with women still underrepresented in “things” jobs. (This Murray shows through a fascinating original analysis of federal job classifications and survey data.) Meanwhile, the most gender-egalitarian countries actually have bigger gaps in certain outcomes, such as the share of women who score highly in STEM (science, tech, engineering, and math) tests but choose not to go into those fields. Some theorize that living in an advanced country allows talented women to do what they want rather than what pays most. Another important fact is that women who score highly in STEM tend to have better language skills, and thus more non-STEM job options, than do men who score highly in STEM. There’s also a growing body of research that looks directly at the effects of male and female hormones and differences in the brains of men and women. Changes in hormone levels tend to exaggerate or reduce sex differences in exactly the direction you’d expect — people injected with testosterone become more impulsive; prenatal exposure to testosterone predicts a child’s future visuospatial ability, autism risk, empathy, and interest in children. As for brains, women have stronger “functional connectivity” in regions involved in emotion processing and social cognition, and there are sex differences in the sizes of numerous brain regions as well. Murray’s discussion of class differences, meanwhile, relies heavily on a much older body of evidence: the research of “behavioral genetics,” especially twin studies. Over a period of decades this work has shown that genes are incredibly powerful, though hardly all-powerful, in shaping how a person turns out within a given society — whether it’s his personality, health, intelligence, or education — while the “shared environment,” which is to say the effect of being raised in the same home (with the same income, neighborhood, parenting philosophy, etc.), is generally weak. Newer methods, involving the DNA sequencing of thousands of subjects, actually pinpoint some of the specific genes that affect important traits, and these methods can even be used to generate a “polygenic score” that predicts from DNA (very imprecisely, but far better than chance) how strongly a person will exhibit a trait. The clear role of genes in life outcomes, coupled with a weak role for the home environment, implies that social class is not just a matter of privilege and oppression and public policy, and not just a matter of personal responsibility and effort, but also heavily a function of natural abilities. This is not exactly a shocking conclusion, though it does challenge some of the more extreme narratives put forth by both Left and Right. One criticism I’ll make of Murray here, though, is that in noting the limits of public policy he could have dealt more thoroughly with various strains of research showing that environments do matter, sometimes a lot, including Raj Chetty’s work on how neighborhoods affect social mobility and Susan Dynarski’s recent study showing that something as simple as promising financial aid up front, rather than later in the process, can make poor kids much more likely to go to a top-tier college. Educational attainment, by the way, is an important trait that is affected by the shared environment quite a bit: 25 percent in a table presented here, though it’s also 50 percent genetic. Lastly, there’s race — the topic that attracted nearly all of the controversy associated with The Bell Curve despite taking up only a minority of its pages. Interestingly, Murray is more circumspect now than he was in that tome a quarter century ago, when he and Herrnstein wrote that it was “highly likely” that part of the gap between blacks’ and whites’ IQ scores was genetic. Here he is more interested in debunking the notion that race is nothing more than a “social construct” that has nothing to do with genes at all. Even on that front he’s pretty timid. Indeed, he begins by agreeing to dispense with the word “race” when talking about genetics, because the word carries so much baggage and the professional geneticists have mostly abandoned it. Instead he goes with “population,” while noting that the ancestral “populations” that geneticists distinguish from one another overlap heavily with commonly used racial categories. Yes, these groups can be identified using nothing but DNA, and yes, there are some important genetic differences among populations: Some less controversial ones affect skin color, malaria resistance, and adaptations for living at high altitudes. In other words, humans have continued to evolve in lots of ways since they spread out across the globe, and different changes have taken hold in different environments. But what about hot-button psychological characteristics such as intelligence? You’d think we’d be getting close to answering that question by now. Recall that we’ve actually identified a lot of genes that affect these traits, and even developed scoring systems that roughly predict from someone’s DNA how he’ll turn out. One imagines you could just apply these techniques to the average DNA profile of each racial group — excuse me, population — and get a simple answer, albeit a tentative one that would become more precise as methods improved and additional influential genes were discovered. But it’s not that easy. For a variety of technical reasons, you can’t apply a single scoring system across multiple populations, at least with current methods. Murray notes that the genetic variants we’ve singled out as playing a role in assorted traits often show up more frequently in some populations than others — a point he makes more painstakingly than he probably needs to, with a series of scatterplots and tabulations — but he admits these gaps are only grist for future, more sophisticated research. His bottom line is not much different from the point made by the prominent Harvard geneticist David Reich in a 2018 New York Times piece: Human populations differ at the genetic level, and we have to prepare for the possibility that these genetic differences substantively affect traits we’d rather they didn’t, but we don’t know the specifics yet. 18 In 1994, Herrnstein and Murray lit the world on fire with a book that made highly controversial claims about IQ, class, and race. Human Diversity’s publicists no doubt hope for a repeat performance; I had to sign a nondisclosure agreement to see the text before the release date. But the result is actually, as Murray himself promises early on, pretty boring for those already familiar with the topics it covers. What it is, is an excellent primer for the uninitiated — at least for a few years, by which point new science will likely have superseded much of the research discussed here. Hopefully the Middlebury kids and their ilk will bother to read it before denouncing it.