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Stretch Genes

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Nat Farbman/Time Life Pictures/Getty Images
Bushman children playing games on sand dunes in the border area between Botswana and South Africa, 1947. The photograph appeared in the Museum of Modern Art’s ‘Family of Man’ exhibition, organized by Edward Steichen in 1955.

1.

Science and science journalism are different things. Though each is valuable, they require at least partly different skills. Science demands unrelenting skepticism about purported facts and theories, and science journalism demands an ability to make the complex clear. Despite my admiration for his work as a journalist, I’m afraid that Nicholas Wade’s latest book reminds us of the risks inherent in blurring the distinction between these endeavors. A Troublesome Inheritance goes beyond reporting scientific facts or accepted theories and finds Wade championing bold ideas that fall outside any scientific consensus.

Wade, now a freelance writer and reporter, is best known for his work as a journalist at The New York Times. He has also written several popular books on biology. The most recent—Before the Dawn (2006) and The Faith Instinct (2009)—focused on evolution in human beings, including the evolution of religion. In A Troublesome Inheritance, Wade maintains this focus on human evolution, though he turns to a far more controversial topic, human races. His goal, he says, is “to demystify the genetic basis of race and to ask what recent human evolution reveals about history and the nature of human societies.” He concludes not only that human races are real but that they probably differ genetically in surprising ways.

Wade’s main claim is that human races likely differ in social behavior for genetic reasons as a result of recent evolution. These slight differences in behavior may, in turn, explain why different sorts of social institutions appear among different peoples:

Institutions are not just sets of arbitrary rules. Rather, they grow out of instinctual social behaviors, such as the propensity to trust others, to follow rules and punish those who don’t, to engage in reciprocity and trade, or to take up arms against neighboring groups. Because these behaviors vary slightly from one society to the next as the result of evolutionary pressures, so too may the institutions that depend on them.

Evolutionary biology might therefore have something to say about why some peoples live in modern states and others in tribal societies, and why some nations are wealthy while others remain mired in poverty.1

The science in A Troublesome Inheritance is mostly inspired by the genomics revolution of the last decade or so. (A genome is the full complement of DNA, the hereditary material, that an individual carries.) This revolution has been, to a considerable extent, a technological and economic one. The high-tech approaches needed to “sequence” a person’s genome—to decipher the three billion units of DNA that make up a human genome—is now sufficiently automated and inexpensive that geneticists have sequenced the genomes of thousands of people from around the world. In the course of this work, results have emerged that throw light on racial differences. Geneticists, Wade says, have been reluctant to talk openly about these results, which are sometimes politically sensitive. He takes up this task here.

A Troublesome Inheritance cleaves neatly into two parts. The first is a review of what recent studies of the genome reveal about our evolution, including the emergence of racial differences. The second part considers the part that genetic differences among races may play in behavior and in the social institutions embraced by various races. These two parts fare very differently.

2.

The key events in human evolution are well known. The lineages that led to human beings and to our closest relative, the chimpanzee, split five to six million years ago. Anatomically modern humans appeared in Africa, the birthplace of our species, about 200,000 years ago. Remarkably, nearly all non-African human beings descend from a small population of people that migrated out of Africa roughly 50,000 years ago. This is an important date for Wade since it places a ceiling on the age of purported racial differences: our ancestors all resided in Africa a mere two thousand or so generations ago.

As people dispersed about the planet, they ultimately settled into the five great “continental races”: Africans (sub-Sahara), East Asians, Caucasians (Europe, the Indian subcontinent, and the Middle East), Australians, and Native Americans. Some of these groups are younger than others (America was peopled only in the last 15,000 years), but this division provides, Wade says, a reasonably realistic portrait of how human genetic diversity is partitioned geographically. Because of their geographic isolation from one another, these groups of human beings necessarily evolved mostly independently over the last tens of thousands of years. During this period of independent evolution, much of what we think of as characteristically human arose, including agriculture and settlement in permanent villages.

So what has study of the human genome over the last decade revealed? Wade’s chief conclusion here is that human evolution has been “recent, copious and regional.” The facts are fairly straightforward. The continental races of human beings differ somewhat from one other at the level of DNA sequence. As Wade emphasizes, these differences are “slight and subtle” but they can nonetheless be detected by geneticists who now have access to many genome sequences from around the planet.

The central fact is that genetic differences among human beings who derive from different continents are statistical. Geneticists might find that a variant of a given gene is found in 79 percent of Europeans but in only, say, 58 percent of East Asians. Only rarely do all Europeans carry a genetic variant that does not appear in all East Asians. But across our vast genomes, these statistical differences add up, and geneticists have little difficulty concluding that one person’s genome looks European and another person’s looks East Asian. To put the conclusion more technically, the genomes of various human beings fall into several reasonably well-defined clusters when analyzed statistically, and these clusters generally correspond to continent of origin. In this statistical sense, races are real.

Why did these genomic differences among peoples appear? There are two main possibilities. The first is that the differences are meaningless. The frequencies of genetic variants can start out the same across several populations and then slowly diverge from one another even when the variants have no effect on Darwinian fitness—defined, roughly, by how many surviving offspring individuals produce. Geneticists call this “neutral evolution.”

The second possibility is that the changes in our genomes were driven by natural selection. According to this hypothesis, the frequencies of genetic variants can diverge among populations because some variants increased the fitness of their carriers, perhaps by increasing their chances of survival in a harsh environment encountered on the particular continent on which they lived. Geneticists have known for some time of cases in which natural selection acted in some, but not other, human populations. Tibetans, for example, appear adapted genetically to life at high altitudes. Until recently, though, we had no way of knowing if such examples of recent natural selection in people were rare or common.

The study of genomes provides new ways to find evidence of natural selection. One approach looks for “selective sweeps.” Natural selection can take a genetic variant that is beneficial but initially rare and drive it to much higher frequency in a population. This process leaves a signal in the genome. Because a whole stretch of DNA that surrounds the beneficial variant will rise to high frequency along with the variant, nearly everyone in the population might end up carrying the same DNA sequence in this part of the genome. Geneticists will thus see a stretch of the genome that shows unusually little genetic variation in a population.

Using this or, more often, related approaches, geneticists have obtained fairly good evidence of natural selection acting on our genomes. Indeed Wade reports that 14 percent of the human genome has experienced recent natural selection. These genomic approaches can’t tell us why natural selection acted in a particular case (was it, say, adaptation to a new parasite?), but they can tell us that these bouts of natural selection were sometimes recent and restricted to particular continents.

Wade’s survey of human population genomics is lively and generally serviceable. It is not, however, without error. He exaggerates, for example, the percentage of the human genome that shows evidence of recent natural selection. The correct figure from the study he cites is 8 percent, not 14, and even this lower figure is soft and open to some alternative explanation.2 And Wade generally assumes that evidence of selection reflects adaptation to the ecological environment, whereas some events might reflect the action of other evolutionary forces like sexual selection, in which individuals compete for mates, not for survival.

Worse, Wade says that biologists realized only in the last few years that natural selection might change a trait by causing slight changes in the frequencies of variants at many genes instead of a large change in frequency at one gene. In fact, the former hypothesis is the traditional view of evolutionary change and is nearly a century old. It would be unfair to suggest that these sorts of mistakes undermine Wade’s main claims in the first part of A Troublesome Inheritance. But they do suggest that he is not the surest guide to a technical literature.

More important, these facts about human genomes don’t seem as little appreciated as Wade lets on. When Henry Louis Gates Jr. sends a sample of his DNA off to find out how much is of African versus European origin—and then acts as host of a PBS miniseries in which he broadcasts the results—it seems hard to maintain that educated people deny that DNA sequences differ subtly among continents. Nonetheless, Wade presents these findings clearly to those who may have missed them.

3.

In the latter half of A Troublesome Inheritance, Wade ventures into far more controversial territory. His claims are, in outline, simple enough.

As human beings evolved over the last tens of thousands of years, the genetic basis of people’s behavior may have changed, just as the basis of their skin color did. Some of these changes may have resulted from Darwinian adaptation to new forms of social life. For example, the “Great Transition” from nomadic life to permanent settlement that began some 15,000 years ago likely produced a profoundly altered social environment: populations grew larger, people interacted with more non-kin, and society became more hierarchical.

In response to this new environment, social behaviors may have changed by natural selection. In some societies, people who were less aggressive or more trusting, for instance, might have prospered under these conditions. Indeed Wade argues that, because the rich could produce more surviving children than the poor once permanent settlements appeared, genes for whatever behaviors underlay their success could spread. “The social behaviors of the elites could thus trickle down into the rest of society” by natural selection.

Crucially, Wade says that “evolution in social behavior has necessarily proceeded independently in the five major races,” reflecting their geographic and thus genetic isolation. The net result of all of this, during settlement as well as other events in recent evolutionary history, is that the continental races might well come to differ genetically in social behavior.

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To Wade, the implications are big. While behavioral differences among races would surely be subtle, they can, he insists, become amplified at the level of entire societies. Slight differences in behavioral predisposition—to cooperation, aggression, trust, propensity to follow rules, and so on—probably pushed different races in directions that led to different social institutions. Indeed the “seeds of difference between the world’s great civilizations were perhaps present from the first settlements.”

Wade devotes much of his book to showing how this evolutionary thesis can help explain all manner of differences among peoples. These include why some peoples are tribal and others modern (modern life requires, among other things, extending trust to non-kin), why some are violent and others less so, why some are poor and others rich, why some are innovative and others conformist, and so on.

Here the book resembles a heavily biological version of Francis Fukuyama’s claims about the effect of social institutions on the fates of states in his The Origins of Political Order (2011). While Wade spends most of his time on these grand differences among peoples, his claims can also get remarkably specific. Why, for instance, do Chinese immigrants to Malaysia and Thailand succeed so often compared to the Malays and Thais themselves? After all,

people are highly imitative, and if Chinese business success were purely cultural, everyone would find it easy to adopt the same methods. This is not the case because social behavior, of Chinese and others, is genetically shaped.3

Wade also thinks that “evolutionary differences between societies on the various continents may underlie major and otherwise imperfectly explained turning points in history such as the rise of the West and the decline of the Islamic world and China.” Here, and especially in his treatment of why the industrial revolution flourished in England, his book leans heavily on Gregory Clark’s A Farewell to Alms (2007). Across these historical turning points, the details differ but the story remains the same: certain peoples were predisposed genetically to behaviors and thus institutions that paved the way for their success, whether, say, economic (the West) or intellectual (the Jews). Other peoples, alas, had other genes.

These are big claims and you’d surely expect Wade to provide some pretty impressive, if recondite, evidence for them from the new science of genomics. And here’s where things get odd. Hard evidence for Wade’s thesis is nearly nonexistent. Odder still, Wade concedes as much at the start of A Troublesome Inheritance:

Readers should be fully aware that in chapters 6 through 10 they are leaving the world of hard science and entering into a much more speculative arena at the interface of history, economics and human evolution.

It perhaps would have been best if this sentence had been reprinted at the top of each page in chapters 6 through 10.

One of the most frustrating features of A Troublesome Inheritance is that Wade wants to have it both ways. At one moment, he will concede that he writes in a “speculative arena” and, at the next, he will issue pseudofactual pronouncements (“social behavior, of Chinese and others, is genetically shaped”). This strategy lets Wade move in a kind of intellectual no-man’s-land where he gets to look like he’s doing science (so many facts about genomes!) while covering himself with caveats that, well, it’s all speculative.4

Which might lead you to wonder: If Wade has little or no hard evidence for his evolutionary thesis, how does he hope to convince his readers to take it seriously? Part of the answer is by offering captivating narratives about how recent human evolution could have played out, as we saw earlier with the transition to permanent settlement. Wade also makes several arguments based on plausibility for the role of genes in behavioral differences among the races and, to a lesser extent, attacks those who have doubted such a role.

One of Wade’s main plausibility arguments involves the difficulty of transferring social institutions from one group of people to another. As he puts it, “one indication of such a genetic effect is that, if institutions were purely cultural, it should be easy to transfer an institution from one society to another.” As we’ve learned, this isn’t always true. For example, “American institutions do not transplant so easily to tribal societies like Iraq or Afghanistan.” And that, it seems, is that. We are to conclude that the differences are probably partly genetic.

This argument is remarkably feeble. Suffice it to say that when we attempt to transfer an institution, say free elections, into another culture, we do not replace one entire culture with another. Instead, we transfer a piece of a culture into an existing one. Is anyone really surprised that this process causes friction? And is it really most plausible to conclude that the source of this friction is differences in genes? What about all those other differences—in history, language, distribution of wealth, religion, educational attainment, ravages of war, arable land, resentment toward perceived invaders, and so on? Among these factors, I suspect that genes are perhaps the one most similar between American and Afghan societies. This isn’t to say that Wade’s argument is necessarily wrong but it is to say that an important feature of a plausibility argument is plausibility.

Another of Wade’s plausibility arguments focuses on stability: “When a civilization produces a distinctive set of institutions that endures for many generations, that is the sign of a supporting suite of variations in the genes that influence human social behavior.” Really? Shouldn’t Wade say that stability “might” be a sign of genes? It’s true that some behaviors or institutions may persist for partly genetic reasons. (Milk-drinking by adults requires lactose persistence, a genetic trait that is more common among cultures that engaged in dairy farming historically.) But it’s also true that some behaviors or institutions persist for purely cultural reasons. The English have used a currency called the “pound” since Anglo-Saxon times. And Western music has been built on a diatonic scale since the Renaissance and probably much earlier. So why doesn’t Wade conclude that differences in currency and musical scale reflect differences in genes?

Conversely, it’s hard to see why profound instability in social institutions doesn’t trouble Wade more. He’s much taken, for instance, with the difference between tribal and modern societies, but one of the most tribal peoples on the planet, the Scots with their clans, are now identified with some of the most modern of ideas and attitudes. Were David Hume and Adam Smith precocious carriers of a mutation that swept Edinburgh?

Similarly, consider the immense institutional differences that distinguish North and South Korea, ones that appeared only decades ago. The people who live north and south of the thirty-eighth parallel have very similar genes, so why do their social institutions differ so dramatically? Wade doesn’t entirely ignore these sorts of examples (he mentions Korea) but it’s unclear why they don’t cause him to doubt the value of his project at least a little. If culture can so easily overwhelm genes—and Wade sometimes seems to concede that it can—why should we care about such pliant genetic predispositions, even if they were real?

Finally, Wade goes on the offensive, attacking those who dare deny that social behavior changed during recent evolution and thus may differ among the races. One target is the evolutionary psychologist Steven Pinker. (Evolutionary psychologists, while acknowledging that human behavior has a partly genetic basis, generally assume that all people share the same predispositions. They then try to explain these “human universals.”) In his 2011 book, The Better Angels of Our Nature, Pinker considers the idea that different groups may behave differently—say, more or less violently—for genetic reasons. He notes that, if true, the idea “could have the incendiary implication that aboriginal and immigrant populations are less biologically adapted to the demands of modern life than populations that have lived in literate state societies for millennia.”

This sends Wade into paroxysms of righteous indignation and he declares that “whether or not a thesis might be politically incendiary should have no bearing on the estimate of its scientific validity.” What Wade doesn’t tell you is that this is what Pinker himself says in his very next sentence: “The fact that a hypothesis is politically uncomfortable does not mean that it is false, but it does mean that we should consider the evidence very carefully before concluding that it is true.”

Pinker then explains why he suspects the genetic hypothesis is wrong. One reason is that no actual data show that the English, for example, are “innately more self-controlled or less violent than the citizenries of countries that did not host an industrial revolution.” Wade responds that while the genes for violence are unknown, murder rates in the industrialized world are lower than those in sub-Saharan Africa, “a difference that does not prove but surely allows room for a genetic contribution to greater violence in the less developed world.” But if the issue is whether people differ innately in a behavioral trait, a material line of evidence cannot be that people differ in the trait. They may easily differ for other reasons. (Do differences in currency have a genetic basis? After all, there are differences in currency!)

Unfortunately, this kind of move is common in A Troublesome Inheritance. The book is filled with suggestive statements like “several social behaviors that economists have identified as obstacles to progress are ones that could well have a genetic basis” and “it is perfectly possible for Jewish populations to have followed a slightly different evolutionary path from Europeans as they adapted to the special circumstances of their history and developed unusual cognitive abilities.” But of course once we know that people differ slightly in DNA sequence, it is “perfectly possible” that any arbitrary difference among them “could well have” a genetic basis. (Do Westerners play chess and Chinese play Go? Well…)

Wade obviously appreciates the distinction between behavior that “could be” genetic and “is” genetic. The problem is that he doesn’t seem particularly interested in hard evidence or even in the prospects that relevant hard evidence could ever be obtained.5

There is, however, another distinction that Wade doesn’t seem to appreciate at all. He’s right that political sensitivities shouldn’t distort scientific truth: the facts are the facts. But as Pinker notes, this doesn’t mean that we shouldn’t be particularly careful when discussing race. History has shown that this is an especially dangerous subject, one that has resulted in enormous abuses. There is nothing unscientific about recognizing this and treading carefully.

At times, Wade’s approach seems almost the opposite. Though he issues the requisite disclaimers about the dignity and moral equality of all peoples, he’s clearly tempted, under the cover of politics-shouldn’t-distort-science, to provoke. Indeed there is a species of bravado here, as though demonstrating that he, unlike others, is tough-minded enough to face unpleasant facts. But surely there is a difference between facing facts that are unpleasant and spinning tales that are improbable.

  1. 1

    Wade writes: “Africa has absorbed billions of dollars in aid over the past half century and yet, until a recent spurt of growth, its standard of living has stagnated for decades. South Korea and Taiwan, on the other hand, almost as poor at the start of the period, have enjoyed an economic resurgence. Why have these countries been able to modernize so rapidly while others have found it much harder?” 

  2. 2

    Wade reports on Joshua M. Akey’s finding that 722 regions of the genome show evidence of positive natural selection in two or more analyses. Akey concludes that this represents 8 percent of the genome. Akey also emphasizes that these data can be difficult to interpret and that other hypotheses, particularly demographic ones, must sometimes be entertained. See Joshua M. Akey, “Constructing Genomic Maps of Positive Selection in Humans: Where Do We Go From Here?,” Genome Research, Vol. 19 (2009). 

  3. 3

    Malaysia and Thailand are near China, so the genetic differences we’re dealing with here are likely subtle indeed. This doesn’t seem to give Wade pause. 

  4. 4

    Speculation, Wade reminds us, has a place in science. But speculation can be productive or not, and the difference involves judgment as well as ensuring that our theorizing remains sufficiently moored to the facts that it cannot drift in arbitrary directions. 

  5. 5

    Geneticists have had an extraordinarily hard time finding genes that make substantive contributions to complex diseases like Type 2 diabetes. This doesn’t bode well, to put it mildly, for finding the genes that allegedly underlie subtle differences in predisposition to middle-class behavioral traits. 

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