No Two Alike

by Judith Rich Harris

  The behavioral geneticists found no main effects (or only negligible ones) of the home environment on personality characteristics related to fearfulness, and yet there are substantial differences in such characteristics between identical twins reared in the same home. Often, one twin is more outgoing—less inhibited—than the other. Why?

  Three red herrings eliminated, two more to go.

  4

  Birth Order and Other Environmental Differences Within the Family

  IT IS NO SECRET that the academic world—the so-called “ivory tower”—is as riven with rivalries and hatreds as any other venue in which humans compete with one another for status, money, and access to desirable mates. Such animosities are the source of a recognized genre of fiction: the academic murder mystery. The murderers in these novels usually turn out to have been motivated by things like ill will associated with tenure decisions or pique that their work was ignored and their rival got the credit.

  Outside of fiction, academic animosities more often involve groups—opposing schools of thought—than individuals. But as you will see later in this chapter, animosities sometimes do occur between individuals, and though I haven’t heard of them leading to murder, they can be very bitter. The case in question involves a dispute over birth order—a dispute between one man who is certain that birth order has deep and lasting effects on personality, a second man who is certain that the first is wrong, and a third who risked his career to ensure that both voices would be heard.

  The fact that this topic arouses strong feelings in some people is not, however, why I’m going to devote most of this chapter to it. Birth order is important to my purposes for two reasons. First, looking for birth order effects on personality—for systematic differences between firstborns and laterborns (or firstborns, lastborns, and middle children)—turns out to be an excellent way to test hypotheses about personality, including some that would be extremely difficult to test in any other way. Second, birth order is—or at least appears to be—a simple thing to study; hence we have loads of data on it.

  Although the developmentalists and the behavioral geneticists could serve as a textbook example, so to speak, of the opposing-schools-of-thought type of academic animosity, they are not as far apart as you (or they) might think. Behavioral geneticists have their cherished beliefs just like anybody else. They too have been reluctant to give up their faith in the efficacy of parental influence. Some behavioral geneticists actually are parents. That’s right, some of them have mated and produced offspring of their own, and I daresay they are as proud of their offspring as the developmentalists are of theirs.

  So when the behavioral geneticists discovered that differences among home environments can’t account for differences in adult personality (which is another way of saying that growing up in the same home doesn’t make siblings more alike), they didn’t straightway announce to the world that “parents don’t matter.” Only one behavioral geneticist, David Rowe, even hinted at that possibility.1 The others searched for ways to reconcile the results of their research with the beliefs they held before the data started coming in.

  The behavioral geneticists realized almost from the start that gene-environment interactions weren’t going to save the day for the home environment. They pinned their hopes on something else: environmental differences within the family. Two children can grow up in the same home and yet have very different experiences there. One might get more of their parents’ affection; the other, more of their parents’ anger. One might be labeled “the thinker” the other might be the athlete of the family. If they differ in age, one has to put up with a pesky younger sibling, the other with a bossy older one.

  Environmental differences within the family exist. They are real and they matter. But can they account for the differences in personality between twins or siblings reared in the same family? Can they explain the variations in behavior that cannot be attributed to variations in genes? Does it all boil down to “Mom always loved you best”?

  Robert Plomin is the world’s most eminent behavioral geneticist. He is a tall, scholarly-looking man with a high forehead and a graying beard (I’ve seen him interviewed on television science programs). Born and raised in the United States, Plomin moved to England several years ago and currently heads a division of a research institute in London.

  Plomin’s 1987 paper with his student Denise Daniels, titled “Why Are Children in the Same Family So Different from One Another?”, laid out the problem boldly and clearly. Biological siblings are not altogether different; they tend to be somewhat alike in personality and intelligence. But their similarity is due to their shared genes. Adoptive siblings aren’t alike at all in adulthood. The evidence, said Plomin and Daniels, doesn’t show that environmental influences are unimportant: it shows that they don’t work the way everyone expected them to. Instead of making siblings more alike, “these environmental influences make two children in the same family as different from one another as are pairs of children selected randomly from the population.”2 These mysterious environmental influences are responsible for about half of the variation in personality among the subjects in behavioral genetic studies—the half I call the unexplained variance.

  It was Plomin’s challenge to academic psychology: If you guys are so smart, let’s see you explain the unexplained variance. He offered some tips: Study at least two children per family and focus on nonshared aspects of their environment—ways in which their environments differ. Don’t bother with the aspects of the environment they have in common; such things have no noticeable effects on personality.

  Researchers geared themselves up for the challenge. Proposals were written and funded.

  In 2001, Robert Plomin, in an article coauthored by two other colleagues (Daniels had left the academic world), issued an update: “Why Are Children in the Same Family So Different? Nonshared Environment a Decade Later.”3 Actually, it was fourteen years later. How time does fly!

  Alas, not much progress had been made in those fourteen years—much less than Plomin had hoped for. This was partly because the developmentalists had largely ignored his advice to study more than one child per family and were still using the same outmoded research methods. But not all of that research funding had been wasted. Two major studies provided relevant, though discouraging, results.

  The first, by Eric Turkheimer and Mary Waldron of the University of Virginia, was a meta-analysis—a statistical technique for combining data from a number of smaller studies in order to get an overall result. Turkheimer and Waldron found forty-three research reports that addressed the question of the unexplained variance. They put them all together and found it was hardly worth the trouble. The nonshared environmental variables measured in the forty-three studies “do not account for a substantial portion of the nonshared variability,” Turkheimer and Waldron concluded.4 Differential treatment by parents—the tendency of parents to treat their children differently—accounted for only 2 percent of the total variance. Differential sibling interaction also accounted for 2 percent. Family constellation variables such as birth order and age differences between children accounted for only 1 percent. The most successful studies were those that looked at outside-the-home variables such as differential interaction with peers or teachers, but even those accounted for only 5 percent. And these meager results came, in many cases, from studies of dubious quality. In some, there were no controls for genetic differences between siblings. In others, multiple measures were collected but the researchers reported only the ones that reached statistical significance.

  The second major study was carried out by a team of researchers who made none of these methodological errors, which is not surprising, given that one of them was Robert Plomin. The study was called NEAD, short for Nonshared Environment in Adolescent Development. I mentioned the NEAD study in chapter 2; it’s the one in which five different kinds of sibling pairs—reared-together identical twins, fraternal twins, ordinary siblings, half-siblings, and step-siblings—were all as
sessed with the same exhaustive battery of tests.

  The study was exhaustive—or do I mean exhausting?—in every respect. The researchers managed to find 720 sibling pairs living in 720 stable two-parent families (no easy task in itself). Each pair of siblings was examined twice, three years apart; the age range was ten to eighteen years. Many measures of the home environment were collected, including the mother’s and father’s behavior toward each sibling and their agreement or disagreement over how to handle them, the relationship between each parent and each child, and the relationship between the siblings. Multiple points of view were obtained and combined; for example, judgments of parental warmth and rejection toward each sibling were made by both parents and both siblings and also by trained observers. There were also many measures of the siblings themselves, including assessments of their antisocial behavior, depressive symptoms, industriousness, autonomy, school performance, sociability, social success, social responsibility, and self-esteem. Because almost all the measures were based on the combined judgments of two or more people, they were exceptionally accurate and reliable.

  As expected, the results showed sizable differences between siblings that could not be attributed either to genes or to aspects of the home environment they shared. The results also showed that the parents did indeed behave differently toward different offspring—again, no surprise. What came as a surprise, and a big disappointment to the researchers, was the complete failure to find any explanation for the sibling differences. Differential parental behavior could be linked to genetic differences between the siblings but not to the nongenetic differences between them. In other words, the results showed that parents were reacting to the genetic differences between their children, rather than causing their children to be different.

  As I mentioned in the previous chapter, a highly active child is likely to be disciplined more harshly than a quiet one. A child born with a predisposition to be disagreeable isn’t treated the same way as one who was born amiable. Genetic predispositions affect the child’s behavior, and the parent’s behavior toward the child is in part a reaction to the child’s behavior. I call such reactions “child-to-parent effects.”5

  Child-to-parent effects explain or contribute to many of the correlations reported by developmentalists: for example, the finding that when parents keep a close eye on their teenagers’ activities, the teenagers are less likely to get into trouble. Monitoring, as it’s called, is not simply a matter of the parents deciding to make the effort; it’s more a question of how much the teenager chooses to tell the parents. Teenagers intent on getting into trouble are unlikely to keep their parents informed about what they’re doing.6 So the developmentalists are right that there’s a correlation, but wrong in assuming that the parents’ behavior is the cause and the teenagers’ behavior the effect. The causal arrow points in the other direction.

  The NEAD data showed that parents do treat identical twins more similarly than fraternal twins, and biological siblings more similarly than step-siblings, but that the differential treatment was due to child-to-parent effects. None of the environmental factors the NEAD researchers looked at could account for more than a negligible fraction of the unexplained variance in the outcomes they measured (antisocial behavior, depressive symptoms, industriousness, self-esteem, and so on). The results of the study were summarized by one of the researchers, David Reiss:

  We can say with confidence that, on the basis of the data we collected, the following family characteristics do not reflect nongenetic, nonshared influences on the adolescent: differential marital conflict about the adolescent versus the sib, differential parenting toward siblings, and asymmetrical relationships the sibs construct with each other…. Given that our very large twelve-year study was designed to identify nongenetic, nonshared factors, this dearth of findings is not only disappointing but galvanizing.7

  Reiss put it more bluntly in an interview with a journalist. “I was shocked,” he confessed.8 David Reiss is a psychiatrist and a psychodynamic family therapist, poor chap.

  The NEAD researchers didn’t expect within-the-family differences in environment to account for all of the unexplained variance. Some of that variance is no doubt due to randomness. Any errors of measurement—personality measures have a fair amount of play in them—would contribute to the unexplained variance. So would developmental noise, the little random zigs and zags that make physical development somewhat unpredictable; even identical twins don’t have precisely identical brains. But the researchers didn’t expect to come up empty-handed. In his 2001 update, Plomin ruefully admitted that it had been shortsighted of them to look for environmental differences only within the home and to ignore outside-the-home experiences:

  Perhaps some sources of nonshared environment can still be found in NEAD…. In retrospect, however, as Harris (1998) has trenchantly pointed out, it seems odd to have looked for differential experiences of siblings solely in the family because siblings live in the same family.9

  But NEAD is only a single study and not necessarily an ideal one. The subjects might have been too old, or the environmental measures too coarse, for the kinds of effects the researchers were looking for. Differential treatment by parents might have its effects in the early years. Or the differential treatment might be subtle. Parents might favor one child over another and yet not show it in ways that the NEAD researchers were able to detect.

  Parents might behave differently, in subtle ways, even to identical twins. There are personality differences between identical twins even in infancy, apparently due to developmental noise. In newspaper accounts of seventeen-month-old conjoined twins Carl and Clarence Aguirre, born attached at the tops of their heads, Clarence is described as lively and cheerful, Carl as quieter and more serious.10 If that’s how their parents see them, aren’t they likely to treat the two babies somewhat differently? In fact, even in the case of twins whose personalities match very closely, aren’t there bound to be little random differences in the experiences they have at home? One gets the upper bunk bed, one gets the lower. One falls into the salmon mousse and ruins their mother’s dinner party and she never entirely forgives him.

  There is a way to answer all these questions: the well-designed birth order study. Like twinning and adoption, birth order is a “natural experiment,” but unlike twinning and adoption it is a common one. The reason it is such a valuable research tool is that the experiences a child has within the home differ systematically depending on his or her ordinal position in the family. Most sources of differences in experiences within the home are either genetic—Mom always loved you best because you were cuter or more amiable than your sibling—or random—developmental noise, bunk beds, salmon mousse. Differences in experiences due to birth order are not genetic (as far as we know, firstborns and laterborns are genetically equivalent on average), and they’re also not the result of developmental noise (whether a neuron zigs or zags doesn’t depend on whether it’s in a firstborn or a laterborn). So looking for personality differences between firstborns and laterborns reared in the same family gives us a way to examine the effects of purely environmental differences, while controlling for the effects of all biological differences, those due to genes and those due to developmental noise.11

  And studying birth order has another advantage over other research methods: it provides a way to separate the effects of within-the-family environmental differences from those of outside-the-family differences. In contemporary societies—at least those not ruled by monarchies—being a firstborn or a laterborn matters only at home. When children go to school, their classmates don’t care (and often don’t know) that Luke is the baby of the family and has four older siblings, or that Michelle has two younger ones whom she helps to take care of. At home, Luke is the smallest among his siblings, but at school he may be one of the largest in his class. This imperfect correlation between the home environment and the outside-the-home environment will prove useful to us.

  The nonsystematic, nonbiological differ
ences in children’s experiences at home—differences of the bunk bed or salmon mousse sort—are impossible to study, because each case is unique and there is no way of putting together enough data for statistical analyses. But the environmental differences due to birth order are predictable, sizable, and go on for a long time. We can study them.

  The differences begin at birth. The firstborn comes into the world and finds herself in the hands of inexperienced parents who hover over her nervously but proudly. Every cry gets an immediate response; every smile is an occasion for fetching the camera. This child is the center of her parents’ life.

  Until her little brother is born. Every firstborn in a family with more than one child has had to experience what some psychotherapists call “dethronement.”12 John Cheever, the novelist and autobiographer who blamed his own depression and alcoholism on his mother’s alleged neglect of him (she worked outside the home), blamed his older brother’s depression and alcoholism on the dethronement that occurred when he, John, was born. Cheever referred to his own birth as the “turning point” in his brother’s life:

  He was happy, high-spirited, and adored, and when, at the age of seven, he was told that he would have to share his universe with a brother, his forebodings would, naturally, have been bitter and deep…. His feeling for me was always violent and ambiguous—hatred and love—and beneath all of this must have been the feeling that I challenged him in some field where he excelled—in the affections of his parents.13