Tuesday, July 01, 2003

The Myth of Inevitable Nature and Reversible Nurture

Nature Via Nurture: Genes, Experience, & What Makes Us Human

By: Matt Ridley

“I intend to tell bizarre stories from the deepest recesses of the genome to show how the human brain is built for nurture,” writes Ridley in Nature Via Nurture: Genes, Experience, & What Makes Us Human. He is not another self-appointed arbitrator, allotting fair shares of the explanation of human personality to the geneticists and the environmentalists. Genes are not the “instead,” he says; they are the “how” of experience, including our own choices. Here are four stories—“bizarre” only until you understand that nature works via nurture—that Ridley tells about our mate choices, intelligence, sexual identity, and parenting.

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Excerpted from Nature Via Nurture: Genes, Experience, & What Makes Us Human  by Matt Ridley. Copyright 2003 by Matt Ridley. Published by HarperCollins Publishers. Reprinted with permission.

It takes a brave author to write another book about “nature and nurture.” English scientist and journalist Matt Ridley tells us why: It is “with the possible exception of the Irish question, the intellectual argument that...changed least in the century just ended.”

The problem, as Ridley poses it in Nature Via Nurture, is that the frequent declarations that the nature/nurture debate is dead have not been followed by a convincing answer to the question: What now? It is not simply that nature and nurture, working side by side, both have a role in shaping human behavior. Instead, writes Ridley, “Every minute, every second, the pattern of genes being expressed in your brain changes, often in direct or indirect response to events outside the body. Genes are the mechanism of experience.”

Nature Via Nurture tells the story of 12 men, from Charles Darwin to Konrad Lorenz, who “put together the chief theories of human nature that came to dominate the twentieth century.” Most were partisans of nature or nurture, but all contributed truth to our understanding of the human mind. Usually, their work is presented as a brief for the nature—or the nurture—side of the debate. Ridley makes the case that each theory actually contributed to the story of how genes are both cause and consequence of our actions, how they shape experience and are shaped by experience, and how “the more we lift the lid on the genome, the more vulnerable to experience genes appear to be.”

Following are brief excerpts that show Ridley making his case for the working of nature via nurture in shaping sex roles and reproductive anatomy, intelligence, sexual identity, and the effects of parenting on children’s personalities.

THE ARGUMENT NOBODY COULD STOP

For more than 50 years, sane voices have called for an end to the debate. Nature versus nurture has been declared everything from dead and finished to futile and wrong —a false dichotomy. Everybody with an ounce of common sense knows that human beings are a product of a transaction between the two. Yet nobody could stop the argument. Immediately after calling the debate futile or dead, the typical protagonist would charge into the battle himself and start accusing others of overemphasizing one or the other extreme. The two sides of this argument are the nativists, whom I will sometimes call geneticists, hereditarians, or naturians; and the empiricists, whom I will sometimes call environmentalists or nurturists.

Let me at once put my cards faceup. I believe human behavior has to be explained by both nature and nurture. I am not backing one side or the other. But that does not mean I am taking a “middle of the road” compromise. As Jim Hightower, a Texas politician, once said: “There ain’t nothing in the middle of the road but a yellow line and a dead armadillo.” I intend to make the case that the genome has indeed changed everything, not by closing the argument or winning the battle for one side or the other, but by enriching the argument from both ends till they meet in the middle. The discovery of how genes actually influence human behavior, and how human behavior influences genes, is about to recast the debate entirely. No longer is it nature versus nurture but nature via nurture. Genes are designed to take their cues from nurture. To appreciate what has happened, you will have to abandon cherished notions and open your mind. You will have to enter a world where your genes are not puppet masters pulling the strings of your behavior but puppets at the mercy of your behavior; a world where instinct is not the opposite of learning, where environmental influences are sometimes less reversible than genetic ones, and where nature is designed for nurture. These cheap and seemingly empty phrases are coming to life for the first time in science. I intend to tell bizarre stories from the deepest recesses of the genome to show how the human brain is built for nurture. My argument in a nutshell is this: the more we lift the lid on the genome, the more vulnerable to experience genes appear to be…

SEX AND ITS EFFECTS

There is one way in which behavior seems to evolve differently from anatomy. In the case of anatomy, most similarities are the result of common descent, or what evolutionists call phylogenetic inertia. For example, human beings and chimpanzees both have five digits on each hand and foot. This is not because five is the perfect number for the lifestyle of both species but because among the early amphibians, one happened to have five digits and most of its myriad descendants, from frogs to bats, have not altered the basic pattern. Some, like birds and horses, do have fewer digits, but none of the apes do.

The same is not true of social behavior. By and large, ethologists have found very little phylogenetic inertia in social systems. Closely related species can have very different social organization if they live in different habitats or eat different food. Distant relatives can have very similar social systems by convergent evolution if they inhabit similar ecological niches. Where two species show similar behavior, it tells you less about their common ancestor and more about the pressures of the environment that shaped them.

A good example is the sex life of the African apes. As primatologists delved further into the lives of apes, they found that alongside the similarities were some intriguing contrasts. These contrasts were thrown into sharper relief by the studies of George Schaller and Diane Fossey on gorillas and Birute Galdikas on orangutans, and the later studies of Takayoshi Kano on bonobos. In the zoo, a chimp looks a bit like a small gorilla. The skeletons of large chimpanzees have been confused with those of small gorillas. In the wild, however, there is a marked difference in their behavior. It all starts with diet. Gorillas are herbivores, eating the stems and leaves of green plants such as nettles or reeds as well as some fruit. Chimpanzees are principally frugivores, seeking out fruit in trees, but adding ants, termites, or monkey meat when they can. This difference in diet dictates a difference in social organization. Plants are abundant but not very nutritious. To thrive on them, a gorilla must spend nearly all day eating and need not move very far. This makes a group of gorillas rather stable and easy to defend. This in turn has tempted male gorillas into evolving a polygamous mating strategy; each male can monopolize a small harem of females and their immature young, driving away other males.

Fruit, however, appears unpredictably in different places. Chimpanzees need to have large home ranges to be sure of finding a fruiting tree. But when a tree is found there is plenty of food to go around, so the animals can share their home range with many other chimps. But because of the large home range, these groups often split up temporarily. Consequently, for the male chimp, the polygamy strategy does not work. The only way to control access to such a large group of females is to share the job with other males. Hence the sexual favors of a troop of chimps are shared among an alliance of males. One becomes the “alpha” male and takes a greater share of the matings, but he does not monopolize.

This difference in social behavior, stemming from a difference in diet, was wholly unsuspected until the 1960s. And it was only in the 1980s that a remarkable consequence became clear. The difference has left its mark on the anatomy of the two ape species. For gorillas the reproductive rewards of owning a harem of females are so great that males which take great risks to get them have generally proved more fecund ancestors than males of a more cautious disposition. And one risk that is worth running is growing to a very large size—even though it takes a lot of food to run a big body. Consequently, an adult male gorilla weighs about twice as much as a female.

Among chimpanzees, males are not under such pressure to be big. For a start, being too big makes it harder to climb trees and also means that you have to spend more time eating. Better to be only a little larger than a female and use cunning as well as strength to rise to the top of the hierarchy. Besides, there is no point in trying to suppress all sexual rivals, because you will sometimes need them as allies to defend the home range.

Among chimpanzees, males are not under such pressure to be big. For a start, being too big makes it harder to climb trees and also means that you have to spend more time eating. Better to be only a little larger than a female and use cunning as well as strength to rise to the top of the hierarchy. Besides, there is no point in trying to suppress all sexual rivals, because you will sometimes need them as allies to defend the home range. However, because most females are mating with lots of males within the troop, the male chimps that most often became ancestors were in the past the ones that ejaculated often and voluminously. The competition between male chimps continues inside the female vagina in the form of sperm competition. Consequently, male chimpanzees have gigantic testicles and prodigious sexual stamina. As a proportion of body weight, chimpanzee testicles are 16 times greater than gorilla testicles. And a male chimp has sex approximately 100 times as often as a male gorilla.

There is a further consequence. Infanticide is common among gorillas, as it is among many primates. A bachelor male infiltrates a harem, grabs a baby, and kills it. This has two effects on the baby’s mother (apart from causing her great, though transient, distress): first, by halting her lactation it brings her back into estrus; second, it persuades her that she needs a new harem master who is better at protecting her babies. And who better to choose than the raider? So she leaves her mate and marries her baby’s killer. Infanticide brings genetic rewards to males, who thereby become more fecund ancestors than males that do not kill babies; hence most modern gorillas are descended from killers. Infanticide is a natural instinct in male gorillas.

But in chimps females have “invented” a counterstrategy that largely averts infanticide: they share their sexual favors widely. The result is that any ambitious male, if he were to start his reign with a killing spree, might be killing some of his own babies. Males that hold back from killing babies therefore leave more offspring behind. To confuse paternity by seducing many males into possible fatherhood, the females have evolved exaggerated sexual swellings on their pink bottoms to advertize their fertile periods.

The size of a chimp’s testicles is meaningless on its own. It makes sense only by comparison with the gorilla’s testicles. That is the essence of the science of comparative anatomy. And having looked at two species of African ape in such a way, why not include a third? Anthropologists are fond of claiming an almost limitless diversity of behaviors in human cultures, but there is no human culture so extreme that it even begins to compare with the social system of either the chimpanzee or the gorilla. Not even the most polygamous human society is exclusively organized into harems that are passed from one male to another. Human harems are built up one by one, so that most males, even in societies that encourage polygamy, have only one wife. Likewise, despite various attempts to invent free-love communes, nobody has succeeded in achieving, let alone sustaining, a society in which every man has repeated brief affairs with every woman. The truth is that the human species has just as characteristic a mating system as any other: characterized by long pair bonds, usually monogamous, but occasionally polygamous, embedded in a large chimp-like troop or tribe. Likewise, however variable testicle size is among men, there is no man whose testicles (as a proportion of body weight) are as small as a gorilla’s or as big as a chimpanzee’s. As a proportion of body weight, men’s testicles are nearly five times as large as gorillas’ and one-third the size of chimpanzees’. This is compatible with a monogamous species showing a degree of female infidelity. The difference between species is the shadow of the similarity within species…

INTELLIGENCE: GENES WILL OUT?

Despite the sweeping successes of twin studies [in demonstrating the heritability of many traits], a few features of human behavior prove to be less heritable. The sense of humor shows low heritability: adopted siblings seem to have quite similar sense of humor, while separated twins have rather different ones. People’s food preferences seems to be barely heritable—you get your food preferences from your early experience, not your genes (so do rats). Social and political attitudes show a strong influence from the shared environment—liberal or conservative parents seem to be able to pass on their preferences to their children. Religious affiliation, too, is passed on culturally, rather than genetically, though not religious fervor.

What about intelligence? The debate about the heritability of IQ has been scarred by controversy since its inception. The first IQ tests were crude and culturally biased. In the 1920s, convinced that intelligence was largely hereditary and alarmed at the thought of excessive breeding by stupid people, governments in the United States and many European countries began to sterilize mental defectives to prevent them from passing on their genes. But in the 1960s came a sudden revolution, as in so many other debates. From then on, even the assertion of heritable IQ led to vitriolic campaigns of denunciation, assaults on your reputation and demands for your dismissal. The first to suffer this treatment was Arthur Jensen in 1969, following his article in the Harvard Educational Review. By the 1990s, the argument that society was segregating itself by assortive mating along intellectual and therefore racial lines—asserted in The Bell Curve by Richard Hernstein and Charles Murray—provoked another wave of rage among academics and journalists.

Yet I suspect that if you took a poll of ordinary people, they would hardly have changed their views over a century. Most people believe in “intelligence”—a natural aptitude or lack of it for intellectual pursuits. The more children they have, the more they believe in it. This does not stop them from also believing in coaxing it out of the gifted and coaching it into the ungifted through education. But they think that there is something innate.

The studies of twins reared apart or together unambiguously support the idea that although some people are good at some things and others are good at other things, there is such a thing as unitary intelligence. That is to say, most measures of intelligence correlate with each other. People who are good at general knowledge tests or vocabulary tests are usually good at abstract reasoning or at tasks that involve completing number series. 

The studies of twins reared apart or together unambiguously support the idea that although some people are good at some things and others are good at other things, there is such a thing as unitary intelligence. That is to say, most measures of intelligence correlate with each other. People who are good at general knowledge tests or vocabulary tests are usually good at abstract reasoning or at tasks that involve completing number series. This was first noticed a century ago by a follower of [Francis] Galton’s, the statistician Charles Spearman, who dubbed the common factor g for general intelligence. Today, a measure of g derived from correlating different IQ tests remains a powerful predictor of how well a child will do at school. There has been more research on g than on any other subject in psychology. Theories of multiple intelligence come and go, but the notion of correlated intelligence just will not go away.

What is g? Something that appears so real in statistical tests must surely have a physical manifestation in the brain. Is it something to do with speed of thought or size of brain, or is it something subtler? The first thing to be said is that the search for the genes of g has been a huge disappointment. None of the genes that are capable of causing mental retardation when broken prove to have any effect on intelligence when altered more subtly. Searching at random through the genes of intelligent people to find ways in which they consistently differ from genes of normal people has so far turned up just one decent statistical correlation (for the IGF2R gene on chromosome 6) and more than 2,000 no-shows. Candidate genes, such as the PLP gene that seems to affect speed of neuronal transmission, have proved capable of explaining only a small degree of reaction time and do not correlate well with g: the speedy-brain theory of intelligence does not look promising.

The one physical feature that does clearly predict intelligence is brain size. The correlation between brain volume and IQ is about 40 percent, a number that leaves much room for the small-brained genius and the big-brained dullard but is still a strong correlation.

The one physical feature that does clearly predict intelligence is brain size. The correlation between brain volume and IQ is about 40 percent, a number that leaves much room for the small-brained genius and the big-brained dullard but is still a strong correlation. Brains are composed of white matter and gray matter. When, in 2001, brain scanners reached the stage that people could be compared for the amount of gray matter in their brains, two separate studies in Holland and Finland found a high correlation between g and volume of gray matter, especially in certain parts of the brain. Both also found a huge correlation between identical twins in volume of gray matter: 95 percent. Fraternal twins had only a 50 percent correlation. These figures indicate something that is under almost pure genetic control, leaving very little room for environmental influence. Gray matter volume must be “due completely to genetic factors and not to environmental factors” in the words of Danielle Posthuma, the Dutch researcher. These studies bring us no closer to the actual genes of intelligence, but they leave little doubt that the genes are there. Gray matter consists of the bodies of neurons, and the new correlation implies that clever people may literally have more neurons, or more connections between neurons, than normal people do. After the discovery of the role of the ASMP gene in determining brain size through neuron number, it is beginning to look as if some of the genes of g will soon be found.

However, g is not everything. Twin studies of intelligence also reveal a role for the environment. Unlike personality, intelligence does seem to receive a strong influence from the family. Studies of the heritability of IQ in twins, adoptees, and combinations of the two have all gradually converged on the same conclusion. IQ is approximately 50 percent “additively genetic”; 25 percent is influenced by the shared environment; and 25 percent influenced by environmental factors unique to the individual. Intelligence therefore stands out from personality in being much more susceptible to family influence. Living in an intellectual home does make you more likely to become an intellectual.

However, these average figures conceal two very much more interesting features. First, you can find samples of people in which variation in IQ is much more environmental and much less genetic than the average. Eric Turkheimer found that the heritability of IQ depends strongly on socioeconomic status. In a sample of 350 pairs of twins, many of whom had been raised in extreme poverty, there emerged a clear difference between the richest and the poorest. Among the poorest children practically all the variability between individual IQ scores was accounted for by shared environment and none by genetic type; in richer families, the opposite was true. In other words, living on a few thousand dollars a year can severely affect your intelligence for the worse. But living on $40,000 a year or more a year makes little difference…

The influence of genes increases and the influence of shared environment gradually disappears with age. The older you grow, the less your family background predicts your IQ and the better your genes predict it. 

The second surprise hidden in the average figures is that the influence of genes increases and the influence of shared environment gradually disappears with age. The older you grow, the less your family background predicts your IQ and the better your genes predict it. An orphan of brilliant parents adopted into a family of dullards might do poorly at school but by middle age could end up a brilliant professor of quantum mechanics. An orphan of dullard parents, reared in a family of Nobel-Prize winners, might do well at school but by middle age may be working in a job that requires little reading or little deep thought.

Numerically, the contribution of “shared environment” to variation in IQ in a western society is roughly 40 percent in people younger than 20. It then falls rapidly to zero in older age groups. Conversely, the contribution of genes to explaining variation in IQ rises from 20 percent in infancy to 40 percent in childhood to 60 percent in adults and maybe even 80 percent in people past middle age. In other words, the effect of being reared in the same environment as somebody else is influential while you are still in that environment but does not endure beyond the period of shared rearing. Adoptive siblings do have partly similar IQs while living together. But as adults their IQs are wholly uncorrelated. By adulthood, intelligence is like personality: mostly inherited, partly influenced by factors unique to the individual, and very little affected by the family you grew up in. This is a counterintuitive discovery exploding the old idea that genes come early and nurture late.

What this seems to reflect is that the intellectual experience of a child is generated by others. An adult, by contrast, generates his or her own intellectual challenges. The “environment” is not some real, inflexible thing: it is a unique set of influences actively chosen by the actor himself or herself. Having a certain set of genes predisposes a person to experience a certain environment. Having “athletic” genes makes you want to practice a sport; having “intellectual” genes makes you seek out intellectual activities. The genes are the agents of nurture…

GIVING BIRTH TO SEXUAL PREFERENCE

The 1990s saw a series of studies that revived the idea of homosexuality as a “biological” rather than a psychological condition, as a destiny rather than a choice. There were studies showing that future homosexuals had different personalities in childhood, studies showing that homosexual men had differences in brain anatomy from heterosexual men, several twin studies showing that homosexuality was highly heritable in western society, and anecdotal reports from homosexual men to the effect that they had felt “different” early in life. On its own none of these studies was overwhelming. But together, and set against decades of proof that aversion therapy, “treatment,” and prejudice entirely failed to “cure” people of gay instincts, the studies were emphatically clear. Homosexuality is an early, probably prenatal, and irreversible preference. Adolescence simply throws fuel on the fire.

What exactly is homosexuality? It is plainly a whole range of behavioral characteristics. In some ways gay men seem to be more like women: they are attracted to men, they may pay more attention to clothes, they are often more interested in people than, say, football. In other ways, however, they are more like heterosexual men: they buy pornography and seek casual sex, for example. (Playgirl’s nude centerfolds of men turned out to appeal mainly to gays, not the intended women.) 

People, like all mammals, are naturally female unless masculinized. Female is the “default sex” (it is the other way around in birds). A single gene, called SRY, on the Y chromosome starts a cascade of events in the developing fetus leading to the development of masculine appearance and behavior.

People, like all mammals, are naturally female unless masculinized. Female is the “default sex” (it is the other way around in birds). A single gene, called SRY, on the Y chromosome starts a cascade of events in the developing fetus leading to the development of masculine appearance and behavior. If that gene is absent, a female body results. It is therefore reasonable to hypothesize that homosexuality in men results from the partial failure of this prenatal masculinization process in the brain, though not in the body.

By far the most reliable discovery about the causes of homosexuality in recent years is Ray Blanchard’s theory of the fraternal birth order. In the mid-1990s Blanchard measured the number of elder brothers and sisters of gay men compared with the population average. He found that gay men are more likely to have elder brothers (but not elder sisters) than either gay women or heterosexual men. He has since confirmed this in 14 different samples from many different places. For each extra older brother, a man’s probability of being gay rises by one-third. (This does not mean that men with many elder brothers are bound to be gay; an increase from, say 3 percent of the population to 4 percent is an increase of one-third.)

Blanchard calculates that at least one gay man in seven, probably more, can attribute his sexual orientation to this effect of fraternal birth order. It is not simply birth order, because having elder sisters has no such effect. Something about elder brothers must actually be causing homosexuality in men. Blanchard believes the mechanism is in the womb rather than the family. One clue lies in the birth weight of baby boys who will later become homosexual. Normally, a second baby is heavier than a first baby of the same sex. Boys especially are heavier if they are born after one or more sisters. But boys born after one brother are only slightly heavier than firstborn boys, and boys born after two or more brothers are usually smaller than first- and second-born boys at birth. By analyzing questionnaires given to gay and straight men and their parents, Blanchard was able to show that younger brothers who went on to become homosexual were 170 grams lighter than younger brothers who went on to become heterosexual. He confirmed the same result—high birth order, low birth weight compared with controls—in a sample of 250 boys (with an average age of seven) who were showing sufficient “cross-gender” wishes to have been referred to psychiatrists; cross-gender behavior in childhood is known to predict later homosexuality.

Like Barker, Blanchard believes that conditions in the womb are marking the baby for life. In this case, he argues, something about occupying a womb that has already held other boys occasionally results in reduced birth weight, a larger placenta (presumably in compensation for the difficulty the baby experiences in growing), and a greater probability of homosexuality. That something, he suspects, is a maternal immune reaction. The immune reaction of the mother, primed by first male fetuses, grows stronger with each male pregnancy. If it is mild, it causes only a slight reduction in birth weight; if strong, it causes a marked reduction in birth weight and an increased probability of homosexuality.

What could the mother be reacting to? There are several genes expressed only in males, and some are already known to raise an immune reaction in mothers. Some are expressed prenatally in the brain. One intriguing new possibility is a gene called PCDH2, which is on the Y chromosome, is therefore specific to males, and is probably involved in building the brain. It is the recipe for a protocadherin. Could this be the gene that wires the bit of brain that is peculiar to males? A maternal immune reaction may be sufficient to prevent the wiring of the part of the brain that would eventually encourage a fascination with female bodies.

Clearly not all homosexuality is caused this way. Some of it may be caused directly by genes in the homosexual person without the mediation of the mother’s immune reaction. Blanchard’s theory may explain why it has proved so hard to pin down the “gay gene.” The main method for finding such a gene is to compare markers on the chromosomes of homosexual men with those of their heterosexual brothers. But if many gay men have straight elder brothers, this method would work poorly. Besides, the key genetic difference might be on the mother’s chromosomes, where it causes the immune reaction. This might explain why homosexuality looks as though it is inherited through the female line: genes for a stronger maternal immune reaction could appear to be “gay genes,” even though they may not be expressed in the gay man himself but only in the mother. 

The absurd distinction between reversible nurture and inevitable nature has now been well and truly buried. Nurture in this case looks just as irreversible as nature, perhaps more so.

But notice what this does to nature versus nurture. If nurture, in this case birth order, causes some homosexuality, it does so by causing an immune reaction, which is a process directly mediated by genes. So is that environmental or genetic? It hardly matters, because the absurd distinction between reversible nurture and inevitable nature has now been well and truly buried. Nurture in this case looks just as irreversible as nature, perhaps more so…

PARENTING AND PERSONALITY

In 1960, a graduate student at Harvard received a letter from George A. Miller, head of the department of psychology, dismissing her from the Ph.D. program because she was not up to the mark. Remember that name. Much later, stuck at home with chronic health problems, Judith Rich Harris took up writing psychology textbooks, books in which she faithfully relayed the dominant paradigm of psychology —that personality and much else was acquired from the environment. Then, 35 years after leaving Harvard, as an unemployed grandmother, having happily escaped academic indoctrination, she sat down and wrote an article, which she submitted to the prestigious Psychological Review. It was published to sensational acclaim. She was deluged with inquires as to who she was. In 1997, on the strength of the article alone, she was given one of the top awards in psychology: The George A. Miller award.

The opening words of Harris’s article were: Do parents have an important long-term effect on the development of their child’s personality? This article examines the evidence and concludes that the answer is no.

From about 1950 onward psychologists had studied what they called the socialization of children. Although they were initially disappointed to find few clear-cut correlations between parenting style and a child’s personality, they clung to the behaviorist assumption that parents were training their children’s characters by reward and punishment, and the Freudian assumption that many people’s psychological problems had been created by their parents. This assumption became so automatic that to this day no biography is complete without a passing reference to the parental causes of the subject’s quirks. (“It is probable that this wrenching separation from his mother was one of the prime sources of his mental instability,” says a recent author, referring to Isaac Newton.)

To be fair, socialization theory was more than an assumption. It did produce evidence, reams of it, all showing that children end up like their parents. Abusive parents produce abusive children, neurotic parents produce neurotic children, phlegmatic parents produce phlegmatic children, bookish parents produce bookish children, and so on. 

Of course, children resemble their parents: they share many of the same genes. Once the studies of twins raised apart started coming out, proving dramatically high heritability for personality, you could no longer ignore the possibility that parents had put their children’s character in place at the moment of conception, not during the long years of childhood.

All this proves precisely nothing, said Harris. Of course, children resemble their parents: they share many of the same genes. Once the studies of twins raised apart started coming out, proving dramatically high heritability for personality, you could no longer ignore the possibility that parents had put their children’s character in place at the moment of conception, not during the long years of childhood. The similarity between parents and children could be nature, not nurture. Indeed, given that the twin studies could find almost no effect of shared environment on personality, the genetic hypothesis should actually be the null hypothesis: the burden of the proof was on nurture. If a socialization study did not control for genes, it proved nothing at all. Yet socialization researchers went on year after year publishing these correlations without even paying lip service to the alternative genetic theory.

It was true that socialization theorists used another argument as well: that different parenting styles coincide with different children’s personalities. A calm home contains happy children; children who are hugged a lot are nice; children who are beaten a lot are hostile; and so on. But this could be confusing cause and effect. You could just as plausibly argue that happy children make a calm home; children who are nice get hugged a lot; children who are hostile get beaten a lot. Old joke: Johnny comes from a broken home; I’m not surprised—Johnny could break any home. Sociologists are fond of saying that a good relationship with parents “has a protective effect” in keeping children off drugs. They are much less fond of saying that kids who do drugs do not get on with their parents.

The correlation of good parenting with certain personalities is worthless as proof that parents shape personality, because correlation cannot distinguish cause from effect. According to Harris, it is patent that socialization is not something parents do to children; it is something children do to themselves. There is increasing evidence that what socialization theorists have assumed were parent-to-child effects are often actually child-to-parent effects. Parents treat their children very differently according to the personalities of the children.

Nowhere is this more obvious than in the troubled matter of gender. Parents who have children of different sexes will know that they treat these children differently. Such parents do not have to be told about the experiments in which adults rough-andtumbled baby girls disguised in blue and cuddled baby boys disguised in pink. But most such parents will also hotly protest that the chief reason they treat their boys differently from their girls is because the boys and girls are different. They fill the boy’s cupboard with dinosaurs and swords, and the girl’s with dolls and dresses, because they know this is the way to please each child. That is what the children keep asking for when in a shop. Parents may reinforce nature with nurture, but they do not create the difference. They do not force gender stereotypes down unwilling throats; they react to preexisting prejudices. Those prejudices are not in one sense innate—there is no “doll gene”—but dolls and many other toys are designed to appeal to predisposing prejudices, just as food is designed to appeal to human tastes. Besides, the parental reaction itself is just as likely to be innate: parents could be genetically predisposed to perpetuate rather than fight gender stereotypes.

I just listened to a radio program about whether boys were better at soccer than girls or whether their parents just pushed them that way. The proponents of each view seemed to agree implicitly that their explanations were mutually exclusive. Nobody even suggested that both could be true at the same time.

Once again, evidence for nurture is not evidence against nature, nor is the converse true. I just listened to a radio program about whether boys were better at soccer than girls or whether their parents just pushed them that way. The proponents of each view seemed to agree implicitly that their explanations were mutually exclusive. Nobody even suggested that both could be true at the same time.

Criminal parents produce criminal children—yes, but not if they adopt the children. In a large study in Denmark, being adopted from an honest family into an honest family produced a child with a 13.5 percent probability of getting in trouble with the law; that figure increased only marginally, to 14.7 percent, if the adopting family included criminals. Being adopted from criminal parents to an honest family, however, caused the probability to rise just to 20 percent. Where both adopting and biological parents were criminals, the rate was even higher—24.5 percent. Genetic factors are predisposing the way people react to “crimogenic” environments.

Likewise, the children of divorced parents are more likely to divorce—yes, but only if they are biological children. Children whose adoptive parents divorce show no such tendency to follow suit. Twin studies reveal no role at all for the family environment in divorce. A fraternal twin has a 30 percent probability of getting divorced if his or her twin gets divorced, about the same correlation as with a parent. An identical twin has a 45 percent probability of divorce if his twin gets divorced. About half your probability of divorce is in the genes; the rest is circumstance.

Rarely has an emperor seemed so naked as after Harris was finished with socialization theory. None of this will come as a surprise to people who have more than one child. Parenting is a revelation to most people. Having assumed you would be the chief coach and sculptor of a human personality, you find yourself reduced to the role of little more than a helpless spectator cum chauffeur. Children compartmentalize their lives. Learning is not a backpack they carry from one environment to another; it is specific to the context. This is not a license for parents to make their children unhappy —making another person suffer is wrong, whether it alters the person’s personality or not. In the words of Sandra Scarr, the veteran champion of the idea that people pick the environments to suit their characters, “Parents’ most important job, therefore, is to provide support and opportunities, not to try to shape children’s enduring characteristics.” Truly terrible parents can still warp somebody’s personality. But it seems likely that (I repeat) parenting is like vitamin C; as long as it is adequate, a little bit more or less has no discernible



About Cerebrum

Bill Glovin, editor
Carolyn Asbury, Ph.D., consultant

Scientific Advisory Board
Joseph T. Coyle, M.D., Harvard Medical School
Kay Redfield Jamison, Ph.D., The Johns Hopkins University School of Medicine
Pierre J. Magistretti, M.D., Ph.D., University of Lausanne Medical School and Hospital
Robert Malenka, M.D., Ph.D., Stanford University School of Medicine
Bruce S. McEwen, Ph.D., The Rockefeller University
Donald Price, M.D., The Johns Hopkins University School of Medicine

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