Identical twins have identical genes; they were formed from the same fertilized egg. Alice James’s theory—heredity—cannot explain the differences between identical twins. Neither can John Cheever’s. Ladan and Laleh went everywhere together; how different could their childhood experiences have been? If one was exposed to family quarrels or misfortunes, so was the other. If one was kept sequestered at home or taken daily to the playground or the marketplace, so was the other. If one was subjected to harsh and early toilet training, would their caregiver have been relaxed and laissez-faire with the other? An overbearing mother, a wimpy or absentee father, parents who get along well, parents who get along poorly, exposure to privation and violence, exposure to books and museums—all these things are shared by identical twins who grow up together. And yet identical twins who grow up together have different personalities.
In a classic British mystery novel such as Dorothy Sayers’ Five Red Herrings,48 the detective spends most of the book examining the suspects one by one and showing why each of them couldn’t have, or wouldn’t have, committed the crime. The one who’s left after everyone else has been eliminated is declared the perpetrator.
In this case, one suspect can be eliminated right off the bat. Nobody believes that “everything is genetic” or that behavior or personality is “determined” by the genes. Clearly, it’s not just genes—it can’t be—that make twins who have identical genes differ in personality.
But I’m not interested only in twins. People also differ from their brothers and sisters, even though siblings have many genes in common and share a home environment. The differences between siblings are a familiar theme in literature. In Shakespeare’s King Lear, two of Lear’s daughters say flattering things about him but the third refuses to do so, to his bafflement and anger. In Beatrix Potter’s The Tale of Peter Rabbit, Flopsy, Mopsy, and Cotton-tail are good little bunnies who do as they’re told, but Peter is always getting into trouble. Such stories are not confined to fiction. You may have read newspaper accounts of a pair of brothers in Boston: one a college president, the other a gangster on the run from the law.49
But I’m not interested only in siblings, either. Twins and siblings are useful, as I will show in the next chapter, because they make it possible to test some theories of personality development. A detective has to have some tools; new tools lead to new methods of detection and often to new conclusions. A suspect found guilty in Shakespeare’s time might today be exonerated on the basis of fingerprint or DNA evidence. Modern methods of testing theories of personality make use of the fact that twins and ordinary siblings are similar in some ways and different in others.
We are all similar in some ways and different in others. I stand with the evolutionary psychologists in believing that humans come with quite a lot of standard equipment. But a good way to find out how the standard equipment works is by seeing how it responds when it receives different inputs from the environment. All normal human babies have a language acquisition device, but which language they learn depends on what their environment provides. It would have been very difficult for psycholinguists to figure out how the language acquisition device works if everyone spoke the same language.
My goal in this book is to show how things we all have in common can make us different. Human nature—the ways in which we are all alike—can lead to human differences. This is not a novel idea; as I said earlier, all theories of personality include an axiom (whether stated explicitly or not) that something is built in. The trouble is that none of these theories can explain why Laleh and Ladan Bijani had different personalities.
2
That Damn Rectangle
THOUGH I WAS often sick as a child and missed a lot of school, the health problems in the first half of my life were intermittent rather than chronic. My healthiest years were in late adolescence and early adulthood. I went to college, spent two years in graduate school, and married one of my fellow graduate students. I managed to produce one child, a daughter. Our second daughter was adopted.
Both daughters were grown, out of the house, and doing well by the time I had the idea that led me to write The Nurture Assumption. But by then my health had worsened and I was unable to travel around interviewing people to gather human interest material for my book. I had to make do with what was on hand. So, with their permission, I used my two daughters as examples of how two siblings reared in the same home can be very different. “Merely corroborative detail,” Pooh-Bah explained in The Mikado, “intended to give artistic verisimilitude to an otherwise bald and unconvincing narrative.”
As a Gilbert and Sullivan fan, I should have known better: the corroborative detail got Pooh-Bah into a lot of trouble. But I didn’t make a big deal about one of my children being adopted; it’s mentioned only once, in a casual way, in the book:
I do not believe that parents have a consistent child-rearing style, unless they happen to have consistent children. I had two very different children—one of them is adopted but the same thing can happen with biological siblings—and used two very different child-rearing styles.1
The press nevertheless pounced on it. They were intrigued by the unusual composition of my family: one biological daughter, one adopted daughter. It didn’t escape their notice that one of the things I talked about in my book was differences between siblings and I just happened to have a perfect example right in my own family. Many journalists jumped to the erroneous conclusion that my rejection of traditional theories of child development, and my proposal of a new theory, were based on my personal experiences as a mother.2
No doubt some theories are based on the theorists’ personal experiences, but mine wasn’t. In fact, it is seldom necessary to think up a new theory in order to account for one’s personal experiences: one can almost always find a way to explain them in terms of existing theories. Neither Alice James, in the nineteeth century, nor John Cheever, in the twentieth, had any trouble explaining their spells of depression in terms of the culturally accepted theories of their times. James blamed heredity; Cheever blamed his childhood family environment. His mother neglected him, he claimed.3
During the years when I was personally experiencing motherhood—the years when my children were young and living at home—my theories were entirely conventional: the culturally accepted theories of the latter part of the twentieth century. My job at that time was writing college textbooks in child development, and there was nothing out of the ordinary about the textbooks I wrote.4 All I was doing was parroting the recognized authorities in the field. It wasn’t hypocrisy: I believed them!
I stopped believing them long after my daughters were launched on their successful adult lives. What led to the sudden and dramatic revision in my thinking was not motherhood but a year spent reading widely in diverse areas of psychology, in preparation for writing another textbook.5 What I noticed in my reading was that something was wrong with the current theories: results that didn’t fit kept turning up. These embarrassing findings weren’t anecdotes or case studies—they came from data based on large numbers of children. Only in data collected from many subjects can one see the forest; otherwise it’s just trees.
The Nurture Assumption was my attempt to explain the puzzling findings turned up by researchers. I put together a lot of evidence showing that parents have no direct influence on how their children turn out and proposed that children are socialized by their peer groups. But socialization is a process that makes children more alike, more similar in behavior to others of their age and gender. If they’re getting socialized, why do they continue to differ from one another in personality? Though group socialization could account for the similarities, my attempt to account for the differences (“differentiation within the group”) was vague and unconvincing.
At least mine wasn’t the only failure. Neither evolutionary psychologists (who focus mainly on the ways that people are alike), nor behavioral geneticists (who focus mainly on the effects of genes), nor developmental psychologists (who focus mainly
on the effects of the environment) have provided an adequate explanation of personality differences. We know that genes cannot account for all the differences, but no one has answered the question of how the environment can make people more alike in some ways and less alike in others. No one has explained why identical twins who grow up together have different personalities.
Here’s an important clue. The unexplained—that is, nongenetic—differences in personality between identical twins reared in the same home are about as wide as the unexplained differences between ordinary siblings. And the unexplained differences between ordinary siblings are about as wide as the unexplained differences between two people of the same sex and about the same age, plucked at random from a city street or college classroom.
It’s an important clue but a baffling one. If this were a mystery novel and the important clue had to do with fingerprints, I wouldn’t have to explain what fingerprints are or how that kind of evidence is obtained. But understanding my clue depends on having some knowledge of the methods and findings of behavioral genetics. In Dorothy Sayers’ mystery Five Red Herrings, the important clue has to do with trains, and the reader is expected to pick up a fair amount of knowledge about the timetables and operating procedures of the trains that chugged their way through rural Scotland in the 1930s.6 I promise you it will be quicker and more useful to learn a little about behavioral genetics.
But I too have five red herrings to let off the hook before I can solve the mystery of individuality. In this chapter I will look closely at the first two suspects: that personality differences are mainly due to differences in environment, or that they’re due to a combination of “nature” (genes) and “nurture” (the part of the environment provided by parents).
The study of individual differences has been part of psychology from the beginning. IQ tests, for example, were first devised about a century ago as a tool for studying differences in intelligence. If every child gave the same response to a question on an IQ test, that question would be useless. The point of an IQ test, or of a personality test, is that different individuals answer the questions differently. Researchers who give IQ tests or personality tests are studying human variation. So are researchers who study children’s aggressiveness by observing them on the playground, or who study their emotional expressiveness by putting them in contrived situations in the laboratory, or who study adolescents’ use of illegal drugs by asking them to fill out questionnaires.
Expressed as numbers, the results may vary widely or fall within a narrow range. The amount of variation will depend on what you’re measuring and how you’re measuring it, and on the sort of subjects you’ve got. There’s likely to be more variation in IQ in a group of basketball players than in a group of physicists, but the physicists will vary more in height. No need to be vague about it: variation can be quantified. The two statistics that are generally used for this purpose—you may already be familiar with them—are the standard deviation and the variance; the first is simply the square root of the second.7 The standard deviation for IQ scores in the general population is 15 IQ points; the variance is 225. It would be a smaller number in the group of physicists. The word “variance” is going to crop up often in this chapter, but all you really need to know is that it’s a way of specifying precisely how widely a given bunch of people vary in some way.
I’ll trouble you with only one other statistical term and it’s one that is in common use: correlation. A correlation is simply a relationship—a statistical linkage—between two sets of measurements. A positive correlation between children’s aggressiveness, say, and how often these children were spanked by their parents would mean that children who received frequent spankings tended to be more aggressive than average and those who received few or none tended to be less aggressive than average. (If children who received frequent spankings tended to be less aggressive than average, the correlation would be negative.) Theoretically, positive correlations can range from zero (meaning that the two sets of measurements are unrelated) to 1.0, but in psychological research they seldom go above .50 and are usually much less than that. Developmental psychologists break open the champagne if they find a correlation as high as .30 between an outcome variable such as aggressiveness and an environmental variable such as frequency of spankings.
The developmentalists’ goal is to explain the differences among their subjects. They want to discover why some individuals are more aggressive, or smarter, or more apt to reveal their emotions, or less apt to use illegal drugs than their fellows. To put it in technical terms, they want to “account for the variance” in the outcome variable they are measuring. They use correlations for this purpose. The trouble is that correlation does not imply causation. If frequency of spankings by parents and aggressiveness in children are found to be correlated, that doesn’t mean that the former caused the latter. It could be the other way round: aggressive children more often try their parents’ patience to the spanking point. Or the correlation could be due to a third factor, genetic influences on personality. If aggressiveness is to some extent inherited, then aggressive parents (who are quick to resort to physical punishment) will tend to have aggressive children. Perhaps all three things are going on at once. There’s no way of telling, just from a correlation.
The goal of the behavioral geneticists is exactly the same as that of the developmentalists: they want to explain the differences among their subjects. They want to account for the variance in the outcome variable they are measuring. They use correlations for this purpose, just like the developmentalists. But there are crucial differences in methodology. The methods used by the behavioral geneticists, unlike those used by the developmentalists, permit them to estimate how much of the variance in outcome can be attributed to genetic influences on personality.
The first discovery to emerge from the behavioral geneticists’ research lets my first red herring off the hook: differences in personality are not due entirely, or almost entirely, to differences in environment. Individuals differ from one another in part because they have different genes. Though this was not really news, its timing was unpropitious: the results started appearing in psychology journals in the early 1970s. At that time there were many people who didn’t want to admit that genes had any effect at all on personality; as a result the first red herring flopped around for years. Environmentalists are still plentiful but—well, I can’t say that the scales have fallen from their eyes, but over time they have become more modest in their claims and less strident in their denunciations.
The behavioral geneticists’ second discovery was news: the environment doesn’t work the way everyone thought. “Nature” was performing as expected, but “nurture” turned out to be a dud. There is variation in personality that cannot be blamed on genes, but neither can it be blamed on the environment that everyone (including me) believed was the most important to a child’s development: the home in which he or she grew up.
Put into numbers, the behavioral geneticists’ research showed that somewhere between 30 and 50 percent of the variance in personality among their subjects could be attributed to “heritability”—the effects of genes.8 Across studies, heritability for personality characteristics averages about 45 percent. The expectation was that the effects of the home environment would account for most of the remaining variance, but it didn’t. In fact, it accounted for only a tiny fraction of the variance and often for none at all.9
The behavioral genetic findings can be summed up like this. Differences in genes cause people to differ in personality, but people also differ for reasons other than genes. Differences in environment of the sort experienced by two children who are reared in different homes appear to have no effect on personality. Being separated in infancy and reared in two different homes does not make identical twins any less alike in personality. Being reared in the same home by the same parents does not make adoptive or step-siblings any more alike.
Though these results puzzled the researchers who discovered them, I noticed
that they dovetailed with other unexpected findings that had been cropping up in research on children. The years of effort devoted to finding something peculiar about the “only child” were a failure; though this child’s home life differs markedly from that of a child with siblings, no consistent differences in social behavior or psychological adjustment have turned up.10 Nor have researchers found any reliable differences between children who spent the daylight hours of their first five years in a day-care center and those who spent that time at home in the company of a parent.11 Children conceived by in vitro fertilization, despite the intense form of parenting they are likely to receive, are indistinguishable from those conceived in the far more common way: accidentally, often in vino.12
In short, whenever a research method is used that controls for, or is not much affected by, the genetic differences between families, the home environment and the parents’ style of child-rearing are found to be ineffective in shaping children’s personalities.
Two red herrings eliminated, right off the bat. The evidence produced by the behavioral geneticists shows that personality differences are not mostly due to differences in environment: genes matter too. The evidence also shows that personality differences are not due to a combination of genes plus the home environment—not due, in other words, to “nature” and “nurture.” And the behavioral genetic evidence doesn’t stand alone. There are many other findings to back it up, reported by developmentalists, social psychologists, anthropologists, and psycholinguists. I put all the evidence together in The Nurture Assumption. My conclusions about personality differences were mainly negative: I showed what the answer couldn’t be. It’s not the way the parents raise the kids.
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