The Blank Slate

by Steven Pinker

  Chapter 19

  Children

  “THE NATURE-NURTURE DEBATE is over.” So begins a recent article with a title—”Three Laws of Behavior Genetics and What They Mean”—as audacious as its opening sentence.1 The nature-nurture debate is, of course, far from over when it comes to identifying the endowment shared by all human beings and understanding how it allows us to learn, which is the main topic of the preceding chapters. But when it comes to the question of what makes people within the mainstream of a society different from one another—whether they are smarter or duller, nicer or nastier, bolder or shyer—the nature-nurture debate, as it has been played out for millennia, really is over, or ought to be.

  In announcing that the nature-nurture debate is over, the psychologist Eric Turkheimer was not just using the traditional mule-trainer’s technique of getting his subjects’ attention, namely whacking them over the head with a two-by-four. He was summarizing a body of empirical results that are unusually robust by the standards of psychology. They have been replicated in many studies, several countries, and over four decades. As the samples grew (often to many thousands), the tools were improved, and the objections were addressed, the results, like the Star-Spangled Banner, were still there.

  The three laws of behavioral genetics may be the most important discoveries in the history of psychology. Yet most psychologists have not come to grips with them, and most intellectuals do not understand them, even when they have been explained in the cover stories of newsmagazines. It is not because the laws are abstruse: each can be stated in a sentence, without mathematical paraphernalia. Rather, it is because the laws run roughshod over the Blank Slate, and the Blank Slate is so entrenched that many intellectuals cannot comprehend an alternative to it, let alone argue about whether it is right or wrong.

  Here are the three laws:

  • The First Law: All human behavioral traits are heritable.

  • The Second Law: The effect of being raised in the same family is smaller than the effect of the genes.

  • 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.

  The laws are about what make us what we are (compared with our compatriots) and thus they are about the forces that impinge on us in childhood, the stage of life in which it is thought that our intellects and personalities are formed. “Just as the twig is bent, the tree’s inclined,” wrote Alexander Pope. “The child is father of the man,” wrote Wordsworth, echoing Milton’s “The childhood shows the man as morning shows the day.” The Jesuits used to say, “Give me the child for the first seven years, and I’ll give you the man,” and the motto was used as the tag line of the documentary film series by Michael Apted that follows a cohort of British children every seven years (Seven Up, Fourteen Up, and so on). In this chapter I will walk you through the laws and explore what they mean for nature, nurture, and none of the above.

  THE FIRST LAW: All human behavioral traits are heritable. Let’s begin at the beginning. What is a “behavioral trait”? In many studies it is a stable property of a person that can be measured by standardized psychological tests. Intelligence tests ask people to recite a string of digits backwards, define words like reluctant and remorse, identify what an egg and a seed have in common, assemble four triangles into a square, and extrapolate sequences of geometric patterns. Personality tests ask people to agree or disagree with statements like “Often I cross the street in order not to meet someone I know,” “I do not blame a person for taking advantage of someone who lays himself open to it,” “Before I do something I try to consider how my friends will react to it,” and “People say insulting and vulgar things about me.” It sounds dodgy, but the tests have been amply validated: they give pretty much the same result each time a person is tested, and they statistically predict what they ought to predict reasonably well. IQ tests predict performance in school and on the job, and personality profiles correlate with other people’s judgments of the person and with life outcomes such as psychiatric diagnoses, marriage stability, and brushes with the law.2

  In other studies behavior is recorded more directly. Graduate students hang out in a schoolyard with a stopwatch and clipboard observing what the children do. Pupils are rated for aggressiveness by several teachers, and the ratings are averaged. People report how much television they watch or how many cigarettes they smoke. Researchers tally cut-and-dried outcomes such as high school graduation rates, criminal convictions, or divorces.

  Once the measurements are made, the variance of the sample may be calculated: the average squared deviation of each person’s score from the group mean. The variance is a number that captures the degree to which the members of a group differ from one another. For example, the variance in weight in a sample of Labrador retrievers will be smaller than the variance in weight in a sample that contains dogs of different breeds. Variance can be carved into pieces. It is mathematically meaningful to say that a certain percentage of the variance in a group overlaps with one factor (perhaps, though not necessarily, its cause), another percentage overlaps with a second factor, and so on, the percentages adding up to 100. The degree of overlap may be measured as a correlation coefficient, a number between -1 and +1 that captures the degree to which people who are high on one measurement are also high on another measurement. It is used in behavioral genetic research as an estimate of the proportion of variance accounted for by some factor.3

  Heritability is the proportion of variance in a trait that correlates with genetic differences. It can be measured in several ways.4 The simplest is to take the correlation between identical twins who were separated at birth and reared apart. They share all their genes and none of their environment (relative to the variation among environments in the sample), so any correlation between them must be an effect of their genes. Alternatively, one can compare identical twins reared together, who share all their genes and most of their environment, with fraternal twins reared together, who share half their genes and most of their environment (to be exact, they share half of the genes that vary among the people within the sample—obviously they share all the genes that are universal across the human species). If the correlation is higher for pairs of identical twins, it presumably reflects an effect of the extra genes they have in common. The bigger the difference between the two correlations, the higher the heritability estimate. Yet another technique is to compare biological siblings, who share half their genes and most of their environment, with adoptive siblings, who share none of their genes (among those that vary) and most of their environment.

  The results come out roughly the same no matter what is measured or how it is measured. Identical twins reared apart are highly similar; identical twins reared together are more similar than fraternal twins reared together; biological siblings are far more similar than adoptive siblings.5 All this translates into substantial heritability values, generally between .25 and .75. A conventional summary is that about half of the variation in intelligence, personality, and life outcomes is heritable—a correlate or an indirect product of the genes. It’s hard to be much more precise than that, because heritability values vary within this range for a number of reasons.6 One is whether measurement error (random noise) is included in the total variance to be explained or is estimated and pulled out of the equation. Another is whether all the effects of the genes are being estimated or only the additive effects: the ones that exert the same influence regardless of the person’s other genes (in other words, the genes for traits that breed true). A third is how much variation there was in the sample to begin with: samples with homogeneous environments give large heritability estimates, those with varied environments give smaller ones. A fourth is when in the person’s lifetime a trait is measured. The heritability of intelligence, for example, increases over the lifespan, and can be as high as .8 late in life.7 Forget “As the twig is bent”; think “Omigod, I’m turning into my parents!”

  “All traits ar
e heritable” is a bit of an exaggeration, but not by much.8 Concrete behavioral traits that patently depend on content provided by the home or culture are, of course, not heritable at all: which language you speak, which religion you worship in, which political party you belong to. But behavioral traits that reflect the underlying talents and temperaments are heritable: how proficient with language you are, how religious, how liberal or conservative. General intelligence is heritable, and so are the five major ways in which personality can vary (summarized by the acronym OCEAN): openness to experience, conscientiousness, extroversion-introversion, antagonism-agreeableness, and neuroticism. And traits that are surprisingly specific turn out to be heritable, too, such as dependence on nicotine or alcohol, number of hours of television watched, and likelihood of divorcing. Finally there are the Mallifert brothers in Chas Addams’s patent office and their real-world counterparts: the identical twins separated at birth who both grew up to be captains of their volunteer fire departments, who both twirled their necklaces when answering questions, or who both told the researcher picking them up at the airport (separately) that a wheel bearing in his car needed to be replaced.

  I once watched an interview in which Marlon Brando was asked about the childhood influences that made him an actor. He replied that identical twins separated at birth may both use the same hair tonic, smoke the same brand of cigarettes, vacation on the same beach, and so on. The interviewer, Connie Chung, pretended to snore as if she were sitting through a boring lecture, not realizing that he was answering her question—or, more accurately, explaining why he couldn’t answer it. As long as the heritability of talents and tastes is not zero, none of us has any way of knowing whether a trait has been influenced by our genes, our childhood experiences, both, or neither. Chung is not alone in her failure to understand this point. The First Law implies that any study that measures something in parents and something in their biological children and then draws conclusions about the effects of parenting is worthless, because the correlations may simply reflect their shared genes (aggressive parents may breed aggressive children, talkative parents talkative children). But these expensive studies continue to be done and continue to be translated into parenting advice as if the heritability of all traits were zero. Perhaps Brando should be asked to serve on grant review panels.

  Behavioral genetics does have its critics, who have tried to find alternative interpretations for the First Law. Perhaps children separated at birth are deliberately placed in similar adoptive families. Perhaps they have contact with each other during their separation. Perhaps parents expect identical twins to be more alike and so treat them more alike. Twins share a womb, not just their genes, and identical twins sometimes share a chorion (the membrane surrounding the fetus) and a placenta as well. Perhaps it is their shared prenatal experience, not their shared genes, that makes them more alike.

  These possibilities have been tested, and though in some cases they may knock down a heritability estimate by a few points, they cannot reduce it by much.9 The properties of adoptive parents and homes have been measured (their education, socioeconomic status, personalities, and so on), and they are not homogeneous enough to force identical twins into the same personalities and temperaments.10 Identical twins are not earmarked for homes that both encourage twirling necklaces or sneezing in elevators. More important, the homes of identical twins who were separated at birth are no more similar than the homes of fraternal twins who were separated at birth, yet the identical twins are far more similar.11 And most important of all, differences in home environments do not produce differences in grown children’s intelligence and personality anyway (as we shall see in examining the Second Law), so the argument is moot.

  As for contact between separated twins, it is unlikely that an occasional encounter between two people could revamp their personality and intelligence, but in any case the amount of contact turns out to have no correlation with the twins’ degree of similarity.12 What about the expectations of parents, friends, and peers? A neat test is provided by identical twins who are mistakenly thought to be fraternal until a genetic test shows otherwise. If it is expectations that make identical twins alike, these twins should not be alike; if it is the genes, they should be. In fact the twins are as alike as when the parents know they are identical.13 And direct measures of how similarly twins are treated by their parents do not correlate with measures of how similar they are in intelligence or personality.14 Finally, sharing a placenta can make identical twins more different, not just more similar (since one twin can crowd out the other), which is why studies have shown little or no consistent effect of sharing a placenta.15 But even if it were to make them more similar, the inflation of heritability would be modest. As the behavioral geneticist Matt McGue noted of a recent mathematical model that tried to use prenatal effects to push down heritability estimates as much as possible, “That the IQ debate now centers on whether IQ is 50% or 70% heritable is a remarkable indication of how the nature-nurture debate has shifted over the past two decades.”16 In any case, studies comparing adoptees with biological siblings don’t look at twins at all, and they come to the same conclusions as the twin studies, so no peculiarity of twinhood is likely to overturn the First Law.

  Behavioral genetic methods do have three built-in limitations. First, studies of twins, siblings, and adoptees can help explain what makes people different, but they cannot explain what people have in common, that is, universal human nature. To say that the heritability of intelligence is .5, for example, does not imply that half of a person’s intelligence is inherited (whatever that would mean); it implies only that half of the variation among people is inherited. Behavioral genetic studies of pathological conditions, such as those discussed in Chapters 3 and 4, can shed light on universal human nature, but they are not relevant to the topics of this chapter.

  Second, behavioral genetic methods address variation within the group of people being examined, not variation between groups of people. If the twins or adoptees in a sample are all middle-class American whites, a heritability estimate can tell us about why middle-class American whites differ from other middle-class American whites, but not why the middle class differs from the lower or upper class, why Americans differ from non-Americans, or why whites differ from Asians or blacks.

  Third, behavioral genetic methods can show only that traits correlate with genes, not that they are directly caused by them. The methods cannot distinguish traits that are relatively direct products of the genes—the result of genes that affect the wiring or metabolism of the brain—from traits that are highly indirect products, say, the result of having genes for a certain physical appearance. We know that tall men on average are promoted in their jobs more rapidly than short men, and that attractive people on average are more assertive than unattractive ones.17 (In one experiment, subjects undergoing a fake interview had to cool their heels when the interviewer was called out of the room by a staged interruption. The plain-looking subjects waited nine minutes before complaining; the attractive ones waited three minutes and twenty seconds.)18 Presumably people defer to tall and good-looking people, and that makes them more successful and entitled. Height and looks are obviously heritable, so if we didn’t know about the effects of looks, we might think that these people’s success comes directly from genes for ambition and assertiveness instead of coming indirectly from genes for long legs or a cute nose. The moral is that heritability always has to be interpreted in the light of all the evidence; it does not wear its meaning on its sleeve. That having been said, we know that the heritability of personality cannot, in fact, be reduced to genes for appearance. The effects of looks on personality are small and limited; blond jokes notwithstanding, not all attractive women are vain and entitled. The heritability of personality traits, in contrast, is large and pervasive, too large to be explained away as a by-product of looks.19 And as we saw in Chapter 3, personality traits can in some cases be tied to actual genes with products in the nervous system. With the comple
tion of the Human Genome Project, it is likely that geneticists soon will be discovering more of those linkages.

  The First Law is a pain in the neck for radical scientists, who have tried unsuccessfully to discredit it. In 1974, Leon Kamin wrote that “there exist no data which should lead a prudent man to accept the hypothesis that IQ test scores are in any degree heritable,” a conclusion he reiterated with Lewontin and Rose a decade later.20 Even in the 1970s the argument was tortuous, but by the 1980s it was desperate and today it is a historical curiosity.21 As usual, the attacks have not always come in dispassionate scholarly analyses. Thomas Bouchard, who directed the first large-scale study of twins reared apart, is one of the pioneers of the study of the genetics of personality. Campus activists at the University of Minnesota distributed handouts calling him a racist and linking him to “German fascism,” spray-painted slogans calling him a Nazi, and demanded that he be fired. The psychologist Barry Mehler accused him of “rehabilitating” the work of Josef Mengele, the doctor who tormented twins in the Nazi death camps under the guise of research. As usual, the charges were unfair not just intellectually but personally: far from being a fascist, Bouchard was a participant in the Berkeley Free Speech Movement of the 1960s, was briefly jailed for his activism, and says he would do it again today.22