The Blank Slate: The Modern Denial of Human Nature
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These attacks are transparently political and easy to discount. More pernicious is the way that the First Law is commonly interpreted: “So you’re saying it’s all in the genes,” or, more angrily, “Genetic determinism!” I have already commented on this odd reflex in modern intellectual life: when it comes to genes, people suddenly lose their ability to distinguish 50 percent from 100 percent, “some” from “all,” “affects” from “determines.” The diagnosis for this intellectual crippling is clear: if the effects of the genes must, on theological grounds, be zero, then all nonzero values are equivalently heretical.
But the worst fallout from the Blank Slate is not that people misunderstand the effects of the genes. It is that they misunderstand the effects of the environment.
THE SECOND LAW: The effect of being raised in the same family is smaller than the effect of the genes. By now you appreciate that our genes play a role in making us different from our neighbors, and that our environments play an equally important role. At this point everyone draws the same conclusion. We are shaped both by our genes and by our family upbringing: how our parents treated us and what kind of home we grew up in.
Not so fast. Behavioral genetics allows us to distinguish two very different ways in which our environments might affect us.23 The shared environment is what impinges on us and our siblings alike: our parents, our home life, and our neighborhood (as compared with other parents and neighborhoods in the sample). The nonshared or unique environment is everything else: anything that impinges on one sibling but not another, including parental favoritism (Mom always liked you best), the presence of the other siblings, unique experiences like falling off a bicycle or being infected by a virus, and for that matter anything that happens to us over the course of our lives that does not necessarily happen to our siblings.
The effects of the shared environment can be measured in twin studies by subtracting the heritability value from the correlation between the identical twins. The rationale is that identical twins are alike (measured by the correlation) because of their shared genes (measured by the heritability) and their shared environment, so the effects of the shared environment can be estimated by subtracting the heritability from the correlation. Alternatively, the effects can be estimated in adoption studies simply by looking at the correlation between two adoptive siblings: they do not share genes, so any similarities (relative to the sample) must come from the experiences they shared growing up in the same home. A third technique is to compare the correlation between siblings reared together (who share genes and a home environment) with the correlation between siblings reared apart (who share only genes).
The effects of the unique environment can be measured by subtracting the correlation between identical twins (who share genes and an environment) from 1 (which is the sum of the effects of the genes, the shared environment, and the unique environment). By the same reasoning, it can be measured in adoption studies by subtracting the heritability estimate and the shared-environment estimate from 1. In practice all these calculations are more complicated, because they may try to account for nonadditive effects, where the whole is not the sum of the parts, and for noise in the measurements. But you now have the basic logic behind them.
So what do we find? The effects of shared environment are small (less than 10 percent of the variance), often not statistically significant, often not replicated in other studies, and often a big fat zero.24 Turkheimer was cautious in saying that the effects are smaller than those of the genes. Many behavioral geneticists go farther and say that they are negligible, particularly in adulthood. (IQ is affected by the shared environment in childhood, but over the years the effect peters out to nothing.)
Where do these conclusions come from? The actual findings are easy to understand. First, adult siblings are equally similar whether they grew up together or apart. Second, adoptive siblings are no more similar than two people plucked off the street at random. And third, identical twins are no more similar than one would expect from the effects of their shared genes. As with the First Law, the sheer consistency of the outcome across three completely different methods (comparisons of identical with fraternal twins, of siblings raised together with siblings raised apart, of adoptive siblings with biological siblings) emboldens one to conclude that the pattern is real. Whatever experiences siblings share by growing up in the same home makes little or no difference in the kind of people they turn out to be.
An important proviso: Differences among homes don’t matter within the samples of homes netted by these studies, which tend to be more middle-class than the population as a whole. But differences between those samples and other kinds of homes could matter. The studies exclude cases of criminal neglect, physical and sexual abuse, and abandonment in a bleak orphanage, so they do not show that extreme cases fail to leave scars. Nor can they say anything about the differences between cultures—about what makes a child a middle-class American as opposed to a Yanomamö warrior or a Tibetan monk or even a member of an urban street gang. In general, if a sample comes from a restricted range of homes, it may underestimate effects of homes across a wider range.25
Despite these caveats, the Second Law is by no means trivial. The “middle class” (which includes most adoptive parents) can embrace a wide range of lifestyles, from fundamentalist Christians in the rural Midwest to Jewish doctors in Manhattan, with very different home environments and childrearing philosophies. Behavioral geneticists have found that their samples of parents in fact span a full range of personality types. And even if adoptive parents are unrepresentative in some other way, the Second Law would survive because it emerges from large studies of twins as well.26 Though samples of adoptive parents span a narrower (and higher) range of IQs than the population at large, that cannot explain why the IQs of their adult children are uncorrelated, because they were correlated when the children were young.27 Before exploring the revolutionary implications of these discoveries, let’s turn to the Third Law.
THE THIRD LAW: A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families. This follows directly from the First Law, assuming that heritabilities are less than one, and the Second Law. If we carve up the variation among people into the effects of the genes, the shared environment, and the unique environment, and if the effects of the genes are greater than zero and less than one, and if the effects of the shared environment hover around zero, then the effects of the unique environment must be greater than zero. In fact, they are around 50 percent, depending as always on what is being measured and exactly how it is estimated. Concretely, this means that identical twins reared together (who share both their genes and a family environment) are far from identical in their intellects and personalities. There must be causes that are neither genetic nor common to the family that make identical twins different and, more generally, make people what they are.28 As with Bob Dylan’s Mister Jones, something is happening here but we don’t know what it is.
A handy summary of the three laws is this: Genes 50 percent, Shared Environment 0 percent, Unique Environment 50 percent (or if you want to be charitable, Genes 40-50 percent, Shared Environment 0-10 percent, Unique Environment 50 percent). A simple way of remembering what we are trying to explain is this: identical twins are 50 percent similar whether they grow up together or apart. Keep this in mind and watch what happens to your favorite ideas about the effects of upbringing in childhood.
THOUGH BEHAVIORAL GENETICISTS have known about the heritability of mental traits (First Law) for decades, it took a while for the absence of effects of the shared environment (Second Law) and the magnitude of the effects of the unique environment (Third Law) to sink in. Robert Plomin and Denise Daniels first sounded the alarm in a 1987 article called “Why Are Children in the Same Family So Different from One Another?” The enigma was noted by other behavioral geneticists such as Thomas Bouchard, Sandra Scarr, and David Lykken and spotlighted again by David Rowe in his 1994 book The Limits
of Family Influence. It was also the springboard for the historian Frank Sulloway’s widely discussed 1996 book on birth order and revolutionary temperament, Born to Rebel. Still, few people outside behavioral genetics really appreciated the importance of the Second and Third Laws.
It all hit the fan in 1998 when Judith Rich Harris, an unaffiliated scholar (whom the press quickly dubbed “a grandmother from New Jersey”), published The Nurture Assumption. A Newsweek cover story summed up the topic: “Do Parents Matter? A Heated Debate About How Kids Develop.” Harris brought the three laws out of the journals and tried to get people to recognize their implications: that the conventional wisdom about childrearing among experts and laypeople alike is wrong.
It was Rousseau who made parents and children the main actors in the human drama.29 Children are noble savages, and their upbringing and education can either allow their essential nature to blossom or can saddle them with the corrupt baggage of civilization. Twentieth-century versions of the Noble Savage and the Blank Slate kept parents and children at center stage. The behaviorists claimed that children are shaped by contingencies of reinforcement, and advised parents not to respond to their children’s distress because it would only reward them for crying and increase the frequency of crying behavior. Freudians theorized that we are shaped by our degree of success in weaning, toilet training, and identification with the parent of the same sex, and advised parents not to bring infants into their beds because it would arouse damaging sexual desires. Everyone theorized that psychological disorders could be blamed on mothers: autism on their coldness, schizophrenia on their “double binds,” anorexia on their pressure on girls to be perfect. Low self-esteem was attributed to “toxic parents” and every other problem to “dysfunctional families.” Patients in many forms of psychotherapy while away their fifty minutes reliving childhood conflicts, and most biographies scavenge through the subject’s childhood for the roots of the grownup’s tragedies and triumphs.
By now most well-educated parents believe that their children’s fates are in their hands. They want their children to be popular and self-confident, to get good grades and stay in school, to avoid drugs, alcohol, and cigarettes, to avoid getting pregnant or fathering a child while a teenager, to stay on the right side of the law, and to become happily married and professionally successful. A parade of parenting experts has furnished them with advice, ever changing in content, never changing in certitude, on how to attain that outcome. The current recipe runs something like this. Parents should stimulate their babies with colorful toys and varied experiences. (“Take them outside. Let them feel tree bark,” advised a pediatrician who shared a couch with me on a morning television show.) They should read and talk to their babies as much as possible to foster their language development. They should interact and communicate with their children at all ages, and no amount of time is too much. (“Quality time,” the idea that working parents could spend an intense interlude with their children between dinner and bedtime to make up for their absence during the day, quickly became a national joke; it was seen as a rationalization by mothers who would not admit that their careers were compromising their children’s welfare.) Parents should set firm but reasonable limits, neither bossing their children around nor giving them complete license. Physical punishment of any kind is out, because that perpetuates a cycle of violence. Nor should parents belittle their children or say that they are bad, because that will damage their self-esteem. On the contrary, they should shower them with hugs and unconditional affirmations of love and approval. And parents should communicate intensively with their adolescent children and take an interest in every aspect of their lives.
A few parents have begun to question the imperative to become round-the-clock parenting machines. A recent cover story in Newsweek entitled “The Parent Trap” reported on the frazzled mothers and fathers who devote every nonworking minute to entertaining and chauffeuring their children for fear that they will otherwise turn into ne’er-do-wells or cafeteria snipers. A similar story in the Boston Globe Magazine with the ironic title “How to Raise a Perfect Child…” elaborates:
“I’m overwhelmed with parenting advice,” says Alice Kelly of Newton. “I read all about how I’m supposed to be providing my children with enriching play experiences. I’m supposed to do lots of physical activity with them so I can instill in them a physical fitness habit so they’ll grow up to be healthy, fit adults. And I’m supposed to do all kinds of intellectual play so they’ll grow up smart. Also, there are all kinds of play, and I’m supposed to do each—clay for finger dexterity, word games for reading success, large-motor play, small-motor play. I feel like I could devote my life to figuring out what to play with my kids.”…
Elizabeth Ward, a Stoneham dietician, has been puzzling over why parents are so “willing to be short-order cooks, preparing two or three meals at a time” in order to please the kids…. [One reason] is a belief that forcing a kid to choose between eating what’s presented or skipping a meal will lead to eating disorders—a thought that probably never occurred to parents in earlier decades.30
The humorist Dave Barry comments on the experts’ advice to parents of adolescents:
In addition to watching for warning signs, you must “keep the lines of communication open” between yourself and your child. Make a point of taking an interest in the things your child is interested in so that you can develop a rapport, as we see in this dialogue:
FATHER: What’s that music you’re listening to, son?
SON: It’s a band called “Limp Bizkit,” Dad.
FATHER: They suck.
… You should strive for this kind of closeness in your relationship with your child. And remember: If worse comes to worst, there is no parenting tool more powerful than a good hug. If you sense that your child is getting into trouble, you must give that child a great big fat hug in a public place with other young people around, while saying, in a loud, piercing voice, “You are MY LITTLE BABY and I love you NO MATTER WHAT!” That will embarrass your child so much that he or she may immediately run off and join a strict religious order whose entire diet consists of gravel. If one hug doesn’t work, threaten to give your child another.31
Backlash aside, is it possible that the experts’ advice might be sound? Perhaps the parent trap is the mixed blessing of scientists’ knowing more and more about the effects of parenting. Parents can be forgiven for carving out some time for themselves, but if the experts are right they must realize that every such decision is a compromise.
So what do we really know about the long-term effects of parenting? Natural variation among parents, the raw material of behavioral genetics, offers one way of finding out. In any large sample of families, parents vary in how well they adhere to the ideals of parenting (if some didn’t stray from the ideal, there would be no point in offering advice). Some mothers stay at home, others are workaholics. Some parents lose their tempers, others are infinitely patient. Some are garrulous, others taciturn; some unreserved in their affection, others more guarded. (As one academic said to me after pulling out a picture of her toddler, “We virtually adore her.”) Some homes are filled with books, others with blaring TV sets; some couples are lovey-dovey, others fight like Maggie and Jiggs. Some mothers are like June Cleaver, others are depressed or histrionic or disorganized. According to the conventional wisdom, these differences should make a difference. At a bare minimum, two children growing up in one of these homes—with the same mother, father, books, TVs, and everything else—should turn out more similar, on average, than two children growing up in different homes. Seeing whether they do is a remarkably direct and powerful test. It does not depend on any hypothesis about what parents have to do to change their children or how their children will respond. It does not depend on how well we measure the home environments. If anything that parents do affects their children in any systematic way, then children growing up with the same parents will turn out more similar than children growing up with different parents.
/> But they don’t. Remember the discoveries behind the Second Law. Siblings reared together end up no more similar than siblings separated at birth. Adopted siblings are no more similar than strangers. And the similarities between siblings can be completely accounted for by their shared genes. All those differences among parents and homes have no predictable long-term effects on the personalities of their children. Not to put too fine a point on it, but much of the advice from the parenting experts is flapdoodle.
But surely the advice is grounded in research on children’s development? Yes, from the many useless studies that show a correlation between the behavior of parents and the behavior of their biological children and conclude that the parenting shaped the child, as if there were no such thing as heredity. And in fact the studies are even worse than that. Even if there were no such thing as heredity, a correlation between parents and children would not imply that parenting practices shape children. It could imply that children shape parenting practices.32 As any parent of more than one child knows, children are not indistinguishable lumps of raw material waiting to be shaped. They are little people, born with personalities. And people react to the personalities of other people, even if one is a parent and the other a child. The parents of an affectionate child may return that affection and thereby act differently from the parents of a child who squirms and wipes off his parents’ kisses. The parents of a quiet, spacey child might feel they are talking to a wall and jabber at him less. The parents of a docile child can get away with setting firm but reasonable limits; the parents of a hellion might find themselves at their wits’ end and either lay down the law or give up. In other words, correlation does not imply causation. A correlation between parents and children does not mean that parents affect children; it could mean that children affect parents, that genes affect both parents and children, or both.