The theory of evolution, as espoused by Charles Darwin in The Origin of Species in 1859, was difficult to accept for religious believers whose assumptions about the world were shattered by it, but Darwin’s The Descent of Man, published 12 years later, posed even greater challenges to people who did accept it, and those challenges continue today. It has often been noted that a disorienting consequence of the Enlightenment was to force people to recognize that humans were not created at the center of the universe in the image of God, but instead on a remote dust-speck of a planet, in the image of mold, rats, dogs, and chimps. For the entirety of recorded history, moral beliefs about humans had been based on the idea that people were in some fundamental sense apart from the rest of nature. Darwin disabused us of that notion once and for all. The scientific and social upheaval that has occurred since Darwin has been an extended process of coming to terms with a unification of humans and the rest of the natural world.

Humans, God, and Nature

I am not a religious person. I grew up in a nonreligious household (my father was ethnically Jewish; my mother was raised Episcopalian). Whatever religious impulses my parents may have had were extinguished when they had to cope with my grandparents’ prejudices about “mixed” marriages in the 1950s, at a time when interdenominational relationships were still rare and scandalous. The world in which I grew up was completely secular, in a typically American way: although we celebrated Christmas with gifts and a tree, and Easter with bunnies and baskets, we never set foot in a church, and Jesus was never mentioned. We didn’t celebrate the Jewish High Holidays either, but my father’s identity as a Jew was stronger than my mother’s as a Christian (she had been alienated by forced church-going as a child), and we lived in a New York suburb where most of my friends were Jews. So I always figured I was Jewish, never mind the opinion of some twelfth-century rabbi who insisted that Jewish identity is matrilineal.

Like many Americans, I am mildly proud of my ethnic heritage and wanted to pass it on to my children. Living in a southern town without much ambient Jewish culture, my Jewish wife and I decided to join the local synagogue. While our kids grew up, we attended regularly. Our synagogue is a lovely place, and I regret none of the time I spent there, but it did not, alas, succeed in fanning the flames of my religious impulses. As a result, I spent a lot of time in an activity that is probably familiar to many secular readers: sitting, politely bored, feigning interest in religious services that had little meaning for me. Stand up, sit down, listen to the prayers. At our synagogue about half of the service was in Hebrew, which I don’t understand, and even the parts in English were addressed to a God that I didn’t, in any remotely literal sense, believe in.

So how to pass the time? Lodged in the rack in the back of the pew in front of me were two books: a prayer book and the Hebrew Bible. It is nothing to brag about, but I had never read a single word of the Bible, Hebrew or Christian. So, figuring I would look like I was participating, I took the Chumash down from the rack, opened it, and started reading chapter 1: the Book of Genesis. I was blown away! No, this isn’t a story of revelation. What knocked me out wasn’t the literal content of what I read, but the poetry. Whoever set those words down the first time did not intend for them to be taken literally. The language itself is beautiful, of course, but what struck me was what the poetry is about: the relationship between humans and the rest of nature.

No one should be subjected to my views on theology, so I will keep this brief. Hopefully readers with a decent religious education will bear with me. What strikes me in Genesis is the repeated assertion that humans are created in the image of God:

What can this mean for the modern agnostic? To me, it suggests that from the outset humans have had a dualistic relationship with the rest of nature. In one respect, created in God’s image we are nature, no different than the inanimate earth and the plants and animals we share it with. There is comfort to be found in unity with the rest of nature, but also despair: is that all we are, a flea on a rock lost in the vastness, here for a moment and then gone forever? So, as told in Genesis, we reassure ourselves with the belief that we are more than that – we experience a dominion over nature, which, ironically, soothes our despair about the meaningless of our existence, but at the same time alienates us from the image of God. In modern terms we are animals, products of evolution indistinguishable from any other, but our actual experience, Darwin and evolution notwithstanding, is of being over and above nature, and thus ironically alienated from it.

That is how the story of Genesis plays out. The first two chapters of Genesis tell the same story twice. The quote above is from chapter 1, in which humans are created by God, male and female together, and given dominion over all of nature. The tone is reassuring. We are special! Little Gods! The mood of the second chapter is very different, however, and much more humbling. God first creates the Garden of Eden, before creating Man (not Woman) from “the dust of the ground.”

This different version of humans has no dominion over nature; they are simply dust into which life has been (temporarily) breathed. The humility that is enforced by mortality comes with an advantage: unified with nature, as a product of dust, Adam is at one with it, as noted in the last line of chapter 2.

You know what happened next. Adam and Eve, created from dust, unashamed of their nakedness, placed in the blissful Nirvana of the Garden of Eden, are given the gift of life as animals, free from work, self-awareness, and disputation. They are prohibited from only one thing: they cannot touch the tree of the knowledge of good and evil, or eat its fruit. Tricked by the serpent, Eve does so anyway, and Paradise is lost forever, starting with their innocent acceptance of their own bodies.

The punishment for our knowledge of good and evil, the only thing about us the theory of evolution can’t take away, is to be exiled forever from nature. Adam and Eve are cast out into a world that sounds a lot like the one we live in every day.

Humans and Beasts

In Tom Wolfe’s novel, A Man in Full, a wealthy developer and horse breeder holds a cocktail party for some equally well-off friends. He decides to entertain his guests by letting them watch the most essential feature of the horse breeding business, the copulation of the mare and the breeding stallion. As the nicely dressed guests sip their drinks, a terrifying scene unfolds when the horses are led in behind a glass panel: both the mare and the stallion are aroused to the edge of violence, genitals engorged, teeth bared, muscles straining, roaring and screeching and crashing into each other. When the exhausted horses are finally led away, the guests are speechless. Someone jokes weakly, “Well, did he offer her a cigarette?”

Why, exactly, was everyone so aghast at this performance? The novel is set in modern times; the guests are educated people who can’t be innocent of where foals come from. They have all, presumably, participated in biologically analogous activities themselves. In fact, the reason for their mortification is obvious. They were stunned into silence in the same way they would have been had they watched the birth of a foal, in its bloody reality, or the butchering of a dead horse for food. The violent, up-close biology of the horses’ copulation forced them to open their eyes to something they already knew perfectly well, rubbed their noses in a truth that people still turn away from, a century and a half after we were first informed of it: humans are animals. Humans are born animals, live as animals, fornicate as animals, breed as animals, and die as animals.

Wolfe’s set piece reminds us that one component of our animal nature is sexual, as documented by Darwin’s younger but contemporary fellow explorer of the far territories of natural science applied to human beings, Sigmund Freud. Whereas human evolutionary theory explains the biological relationship between humans and the rest of living nature, Freudianism is a psychological exploration of the realization that people are animals like any other. Indeed, Freud offers the best way to make sense of the comically misplaced shock of the sexually sophisticated partygoers: knowledge of the homology between their sexuality and that of beasts had been repressed, and they were experiencing its uncomfortable return.

The first great psychological challenge posed by Darwinism is the problem of human nature, a word that we will encounter again and again in this book. We accept that horses, although adaptable in many ways, have physical and behavioral characteristics that follow from their biological evolution and physical form. In fact, that is all that horses have, innocent as they are of the fruit of the tree of knowledge. An enlightened commitment to science would seem to suggest that people, who are no less subject to evolution than horses, can’t be any different. Humans, as Darwin and Freud force us to recognize, are by our nature social, sexual, and potentially aggressive; our family and group structures have obvious origins in the social structure of the primates from whom we most immediately evolved. What are we to make of this, either as ordinary individuals or as scientists? The nature side of nature–nurture is about the relationship between humans and nature; it is about the problem of Genesis. This is why nature–nurture is a debate, and why it never goes away: post-Darwinian people know that we are one with nature, completely so in a scientifically literal sense, but we are also unable to give up the idea that we are apart from it. This debate rages not only between people but within them, which is what has kept the Freudians in business for 150 years.

Biological Variation and Human Equality

Like the biblical notion that humans are created in the image of God, the second sentence of Thomas Jefferson’s Declaration of Independence is as poetically true as it is empirically false. The idea has obvious roots in the idea that humans were created in the image of God: “We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights … .” Presumably our rights are inalienable because we are created in the image of God, but what about being created equal? Godlike beings may be equal in their divinity, but a simple look around is enough to convince anyone that people aren’t literally equal. Although he died 30 years before the publication of the Origin of Species, Jefferson was an accomplished botanist who certainly knew about biological variation. He was, of course, also a slave owner whose commitment to the real-world equality of human beings was incomplete at best.

Jefferson perfectly embodied this paradox of human biological and moral equality. For all his imperfection and hypocrisy, Jefferson’s famous sentence meant that people are morally and politically equal despite their obvious differences. Embedded in the Declaration, addressed to a world that assumed the natural supremacy of the white male ruling classes, Jefferson’s assertion was literally revolutionary. Was it possible to create a society based on equality, given the hard empirical fact of difference? America and its struggles over the next 250 years are a testimony to the urgency and difficulty of the question.

Human Science

After Darwin and Freud, with God dead, human beings became objects of scientific investigation. Human science has been the single greatest revolution in human culture; much of it has been unambiguously successful and entirely uncontroversial. Scientific understanding of human anatomy and physiology was already well underway before Darwin provided the full evolutionary context. Many aspects of human evolution sit comfortably in the domain of biology. Since Darwin, the study of how human beings evolved from primates in sub-Saharan Africa, and then migrated from Africa and steadily populated the rest of the globe, has been filled out to a remarkable degree. The evolution of human physical characteristics and their analogies to earlier primates, which were so scandalous when Darwin first pointed them out, are obvious to the modern evolution-aware sophisticate. Thanks to Freud, we can even swallow hard and admit that as a matter of biology, humans fornicate and reproduce pretty much the same way as other animals. As a means of pumping blood, the human heart is like a pig’s heart, and can be studied and understood in the same way.

Nevertheless, a deep paradox underlies our attitudes about human biology. I presented the idea of applying science to humans in a way that made it easy to accept – of course there is such a thing as human anatomy, physiology, and medicine, and of course these sciences must be understood in the context of evolution. But even though all modern Darwinists endorse the idea that humans are animals and can be studied scientifically as animals, no one believes it. Reports of God’s death are greatly exaggerated. The scientific practices that are applied to nonhuman animals are, notwithstanding their practical justification and the stringent ethical protections that are applied to them, positively gruesome. We routinely “sacrifice” animals to see what happens to their brains following some experimental manipulation. We raise them in cages or worse. We dissect them to understand their internal physiology. We force-breed them to study their genetics or knock out genes to study their development. We eat them.

I mention this not because there is something fundamentally immoral about animal research, which, as I say, is generally well justified and closely regulated. But even thinking about any of these practices being applied to human beings feels abhorrent, a definition of Mengele-level genocidal abuse. Nevertheless, there are two important points to see plainly. The first is that a moral refusal to experiment on human beings stands in opposition to a scientific conviction that humans are ordinary animals. The second is that those same ethical proscriptions put severe, intractable limits on the extent to which humans can ever come into the light of scientific explanation. These principles go together: we do not experiment on humans because we believe (whether in a religious or secular sense) that humans are sacred; we believe humans are sacred because, in our experience, we exist outside the deterministic domain of natural science. Humans remain outside that domain because we can’t – mustn’t – create the inhuman conditions that might show otherwise. We can either maintain a special proto-scientific place for human freedom and morality, or we can pursue human science to its carnivorous, fascist, deterministic end; we can’t have both. To be perfectly clear, this state of affairs is both necessary and good. It might be the definition of “good.” I explore it more fully in the last chapter of the book.

Social Science

The theory of evolution, combined with Freud and the relaxation of religion’s stranglehold on human thought, gave birth to a new kind of science, applied to the behavior of human beings. As soon as it was established that humans were part of nature, humble keepers of nature’s garden rather than godlike masters of it, it became possible to ask about the implications of our place in nature for the way we understand ourselves. A re-examination of the human relationship with nature, therefore, was one and the same as a new examination of the relation of human behavior to natural science. If people were part of nature, objects in the physical world and evolved instances of biology, wasn’t it just a matter of time and the inexorable progress of science before humans, including human beliefs, human relationships, and human cultures, would come to be understood and controlled in the same way that the once-mysterious physical world had been tamed by the likes of Newton, and that the human body was rapidly being tamed by the anatomists and physiologists of the day?

For social theorists at the end of the nineteenth century, there was every reason to think that the application of natural science to the behavior of human beings and their collective cultures was both inevitable and salutary. The “civilization” that was so valued by European thinkers of the time had been made possible by natural science applied to the physical world. What was the industrial revolution other than an application of physical science to human commerce? What was medicine other than an application of the scientific recognition that human bodies are no different in principle from the bodies of farm animals? From the point of view of the social theorists of the day, science had brought nothing but good into human existence.

Having embarked on a science of human behavior, how far are we willing to extend our evolved relationship with primates, rats, and insects? If our arms, legs, genitals, and DNA have obvious analogs in other animals, what about our mating rituals, cultures, political systems, arts, and wars? What of morality and religion? Have these evolved from lower animals in the same sense as bipedal locomotion? The answers to such questions are not nearly as clear, even today. This new kind of science, which we call social science, embodies the full paradox of the relationship between humans and nature. On the one hand, the questions it poses are the most profound that people know how to ask: right and wrong, equality and inequality among individuals and cultures. On the other hand, social science has proved unable to replicate the relative epistemological certainty, moral neutrality, and instrumental utility of the natural sciences. I do not mean to imply that natural science has achieved (or can achieve) either perfect certainty or moral neutrality. No science is independent of its human instantiation.

Recognition of the less-than-absolute status of scientific knowledge, however, should not blind us to the enormous gap between what natural science has taught us and what we have learned from studying ourselves. The difference is not metaphysical, but practical. No scientist has access to unfiltered “reality,” but the fact that we are not allowed to dissect the brains of infants randomly assigned to different levels of alcohol exposure comes with a price in what can be understood about human development. Social scientists thus find themselves in the awkward position of addressing the most difficult questions with the bluntest instruments. This paradox has led social science to a century and a half of uneven and controversial progress. These ideas are explored further in Chapter 10.

Darwin and Galton

By the time Darwin published The Descent of Man, his half-cousin, Reference GaltonFrancis Galton, had preceded him with his own take on the implications of evolutionary theory for the understanding of human beings, titled Hereditary Genius. The two men and the books they wrote were vastly different. In The Descent of Man, Darwin adopted his usual stance of the humble naturalist, that is to say, nature-alist, in this case one who has stumbled upon the human species. Observations are marshaled painstakingly as he builds the case for human descent from primates. For someone who had already spawned the greatest intellectual revolution in modern history, Darwin barely pauses to wonder about the great moral and social questions that will follow from his discovery. Humans, like all species, descended from earlier species, according to the laws (most of them unknown at the time) of heredity and selection. Why this would be true – what it means that it is true – was simply not the sort of question Darwin was inclined to think about. Darwin’s intellectual humility, finally, secured his sainthood in the modern scientific firmament, with Einstein his only possible rival.

Francis Galton was exactly the opposite. Whereas Darwin dedicated his life to the objective, selfless observation of nature and humankind’s oneness with it, Galton set himself the gargantuan task of investigating and understanding the meaning of evolution for human individuals and societies. Hereditary Genius is not a book about evolution or human biology: it is about the consequences of evolution for understanding the structure of human society and the place of individuals in it. If Darwin was a naturalist as both a biologist and philosopher, Galton was, sui generis, a social scientist. He was, famously, a polymath. He was one of the first Europeans to explore Africa; he made contributions to meteorology; he was the first to undertake the scientific study of fingerprints. His genius was for inventing the tools he needed – research paradigms, methods of data collection, essentially the totality of twentieth-century social science statistics – on the fly as he needed them.

Galton was motivated neither by the broadest philosophical questions about human activities nor by the more literally biological ones about human bodies. What interested Galton was the applied analysis of Jefferson’s proposition about equality, in a more or less evolutionary framework. Variability is a crucial part of the theories of evolution and genetics. Individual organisms differ for reasons related to the genes they carry, and in a given environment those differences are related to the probability that an individual will survive, reproduce, and thereby pass the relevant genes to the next generation. As a result, differences in the observable characteristics of organisms (referred to as phenotype) are related to differences in their genetic makeup (referred to in the abstract as genotype). Other differences among the phenotypes of organisms are related to characteristics of the environment in which they exist; these differences are not passed down biologically from one generation to the next. (Many nongenetic differences, like language, are still passed down, via culture or other environmental features that persist across generations. This is one of many ways in which the distinction between biology and environment is less supportable than it appears at first.)

Most influentially in the long run, Galton undertook the first scientific attempt to apply evolutionary and genetic theory to the complex behavioral phenotypes of human beings. Galton recognized that there was a link between the implied moral proposition in Jefferson’s declaration of human equality and the biological processes of fitness, survival, and evolution that had been uncovered by Darwin. In so doing, Galton invented the nature–nurture debate in its modern form. Galton borrowed the phrase nature–nurture from Shakespeare’s The Tempest, in which the protagonist Prospero reflects negatively on the character of the (dark-skinned and non-European, but never mind) Caliban: “a born devil, on whose nature/Nurture can never stick.” Galton, ever the intellectual bricoleur, happily adopted the term: “The phrase ‘nature and nurture’ is a convenient jingle of words, for it separates under two distinct heads the innumerable elements of which personality is composed. Nature is all that a man brings with himself into the world; nurture is every influence without that affects him after his birth.” Galton set himself the task of determining how much of the differences among humans was inherent, genetic, the result of “nature,” as opposed to environmental, cultural, or nongenetically familial, the result of “nurture.”

Galton’s question was about a different aspect of the nature–nurture problem than I have discussed so far. Questions about relations between human beings and nature, God if you like, biology and evolution, are about universals. Such questions apply equally to everyone; they are about human beings as a collective, as a species. Scientists refer to such characteristics as species-typical. Galton was concerned with another side of the evolutionary challenge: not about universal human characteristics, but instead about the origin and meaning of human differences. Almost any question about organisms has both a species-typical and an individual-difference aspect. “Why are people about five and a half feet tall?” is a species-typical question; “Why are some people taller than others?” is an individual-difference question. The same is true for behavior. “Why are people sociable?” is species-typical. “Why are some people more extroverted than others?” is about individual differences. We can see right away that species-typical and individual-difference aspects of a single characteristic can have quite different explanations. In a famous example that we will revisit later, the reasons why people in general have two arms have to do with our evolutionary descent from symmetrical organisms over millions of years, and more recently from mammals and primates. The explanation of why some individuals don’t have two arms involves either congenital disease or accidents.

If Galton was going to investigate the relative contributions of nature and nurture to human behavioral or cultural differences, the first thing he needed to do was to invent an empirical method for studying it. The problem does not have an obvious answer. Decisive experiments that we might be able to conduct in plants or nonhuman animals are obviously impermissible in humans. Many aspects of the relationship between people and biology, as we have seen, are philosophical or even theological. There is no obvious scientific experiment to conduct that would inform us of where our notions of right and wrong come from or whether humans have a moral standing in the natural world that can be distinguished from that of other living beings. Freud, for example, although he made profound and lasting theoretical contributions to the self-comprehension of humans, ultimately failed in his most cherished enterprise, that of turning psychoanalysis into a natural science, to the great (albeit not entirely deserved) detriment of his contemporary reputation.

Reading The Descent of Man and Hereditary Genius today (both are in the public domain and available for free online), one cannot fail to be struck by how different they are. Even setting aside the size of its scientific contribution, The Descent of Man impresses with Darwin’s intellectual humility and his grace as a science writer. Descent is scrupulously detailed, and in large portions not directly concerned with humans’ evolutionary origins, but in its quiet, scholarly accumulation of natural evidence it remains a pleasure to read, even for the nonspecialist. Along with everything else it accomplished, The Descent of Man is a masterpiece of scientific literature, a quality it shares with Freud’s best writing. Galton’s Hereditary Genius, in contrast, is notable for three characteristics: its bracing but mostly ill-deserved audacity, its casual racism and classism, and the butt-numbing boredom it induces in a modern reader.

Galton’s New Science

We will get to Galton’s exaggerated claims and enthusiastically regressive scientific politics later. For now, I want to dwell a moment on the tediousness, because it is a quality it shares with much of the modern social science it spawned. Galton’s first mission was to study the familial clustering of what he called “eminence,” meaning the accumulation of wealth and professional accomplishment, among the upper classes of Victorian society. The statistical methods that he and his followers would eventually develop for this purpose were in their early phases, but there was still a tremendous amount of work to do in the identification of the relevant individuals, classification of their family relations, and quantification of their various accomplishments. It cannot be denied that Galton undertook this task with immense energy, cataloging the biographical and professional details of hundreds upon hundreds of barristers, scientists, and artists. To a modern social scientist, the book induces a familiar mix of admiration for the work that must have been involved, with ever-increasing doubt that all this effort could possibly have been worth it, given its distant and ill-considered theoretical connection to the questions Galton presumed to be pursuing. After 150 years of social scientific test designing, questionnaire writing, interview recording, blood collecting, and finally gene sequencing, it all has an eerily familiar ring: science as stamp collecting, justified by the technological diligence it requires, not the scientific insight it provides.

Nature and nurture comprise the most profound questions humans can ask about themselves: what is the place of human beings in the natural order, and what is the place of an individual human being among the billions who exist, have existed, and will exist in the future? Do we, godlike, create ourselves, or are we as subject to the deterministic laws of nature as the dust from which we arose? The nature–nurture debate is about mortal humans’ relation with the immortal nature. The remainder of this book will explore those questions.

For better or for worse, Galton’s genius was for inventing empirical methods that could be applied to complex theoretical problems, and for turning great but ineffable questions into soluble, albeit somewhat trivialized, matters of quantitative tabulation. His idea was that the genetic and environmental components of differences, what came to be called “the relative contributions of nature and nurture,” could be quantified, and that their contributions represented an empirical answer to Jeffersonian questions about equality and inequality. So, for example, people differ in height. Why? Presumably some of the reasons have to do with heredity. Other reasons are environmental – some people may have contracted a debilitating disease as children or been subjected to malnutrition. Galton developed methods for quantifying such things, and in so doing originated much of what we still think of as mathematical statistics.

It turns out that there are enormous complications to Galton’s methods, especially about whether genetic and environmental causes of human differences can be meaningfully separated in this way; these complications will take up large sections of this book. In fact, Galton’s science of individual differences was in many ways the inverse of Freud’s: if Freudianism is an astute theory that struggles to find a rigorous empirical basis, Galtonism is a toolbox full of ingenious empirical methods that have never been able to identify precisely what question they are supposed to answer. Galtonism is a set of well-made hammers, and in the absence of a clear purpose they have had the distracting tendency to turn a lot of complex human problems into nails.

The often-feckless activity of hammering empirically on complex human behavior is what we today call social science, and it was invented single-handedly by Francis Galton. Galton said, Reference Pearson“Whenever you can, count.” Galton’s compulsion to quantify is usually credited as his great accomplishment, but it has done more harm than good in the long run. It has led to a modern state of affairs characterized by a famous quote attributed to the educational philosopher Robert Hutchins: “A social scientist is a person who counts telephone poles.” Unfortunately, the version of the nature–nurture debate with which most people are familiar owes much more to Galton and telephone pole counting than it does to the Book of Genesis, Jefferson, Darwin, or Freud. The modern nature–nurture debate has been boiled down to a sterile empirical question about whether “genes” or “environment” are more important for the explanation of human differences. For almost any trait that people care about, that question has no interesting answer. Most of the prodigious effort that has gone into answering it has been wasted.

Fortunately, however, the nature–nurture debate is about something more consequential than the futile empirical science with which it has often been associated. It is called the nature–nurture debate, and not the gene–environment debate, for a reason: nature is much greater than genetics or even evolution, and nurture, with its connotations of mothering, is more central to our experience than the rat-lab notion of the “environment.” There is a lot more history and terrain to cover before we can get to such questions.