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Judith Rich Harris
Frank Sulloway is right when he says that a younger sibling would be ill advised to punch his older brother in the nose: the punch might be returned, and older kids punch harder than younger ones. But the same younger sibling who learns through hard experience to stay his hand at home may nonetheless become the bully of the playground, if he happens to be larger or stronger than other children of his age. As I show in my forthcoming book The Nurture Assumption, the strategies children work out at home for getting along with their parents and siblings are likely to be useless in the world outside their home. That is why children's behavior differs systematically in different social contexts. And that is why psychologists looking for birth order effects in modern populations have again and again failed to find them.
It was different in the old days. In former times, children spent most of the day in the company of their siblings, so a younger sibling might spend his entire childhood in the shadow of an older brother. And the rule of primogeniture meant that a child's birth order determined his status not only within his family but in the society as a whole, from the cradle to the grave.
Today, children interact with their siblings mainly at home. Outside the home they spend most of their time in the company of same-age peers. Developmental psychologists have looked for, and have not found, a carryover of behavior from sibling relationships to relationships with peers. Children who fight like cats and dogs with their siblings are not more likely to have troubled peer relationships. A child who submits to an older sibling at home may be a leader in her nursery school classroom. Sure, children learn things at home. But they learn new things, different things, when they go out. And it's what they learn Out There that they carry with them to adulthood, because Out There is where they are destined to spend the rest of their lives.
The idea that birth order has important and persistent effects on personality has been repeatedly debunked by careful reviewers of the data -- reviewers without a theoretical ax of their own to grind. And yet people go on believing in the power of birth order. I attribute the persistence of this belief to what I call "the nurture assumption": the assumption that what makes children turn out the way they do, aside from their genes, is the way their parents bring them up. Since it is clear that parents do not treat all their children alike, and equally clear that firstborns are treated differently from laterborns (the oldest is given more responsibility, the youngest more affection), the nurture assumption predicts that order of birth should leave permanent marks on the children's personalities. Only it doesn't -- at least not in modern populations. Or if it does, the effects are so small and unreliable that they are of no practical importance. Birth order effects cannot, for example, explain the fact that children reared in the same family do not turn out alike: at most they can account for only a tiny fraction of the environmentally derived variation in personality.
Sulloway is right that birth order is a "systematic source of differences in family environments"; he is right that siblings have a tendency to diversify. They may get interested in different things and choose different careers. Their birth order unquestionably affects their relationships with each other and with their parents; it affects the way they behave at home. What it does not affect is their adult personality, measured outside the home or judged by people who are not members of the family.
In his Edge interview, Sulloway gives the impression that self-report personality tests -- the kind where people answer questions about themselves -- are worthless and that the psychologists who construct them are naive enough to take the subjects' statements about themselves at face value. The truth is that personality tests are sophisticated devices that have been honed and improved over time. They are examined for internal consistency and checked against other sources of information; test items that don't work are eliminated. No single item on the test can do the job unaided; the scorers of these tests are looking for *patterns* of responses. The "Big Five personality dimensions" that Sulloway talks about are a product of the same tests that he dismisses when they produce results not to his liking.
Sulloway asks but one question of his historical subjects: Do they or don't they believe in evolutionary theory, or phrenology, or the Protestant Reformation? It's a test consisting of a single item. How well can we judge someone's personality by his answer to a single question?
But Sulloway has more than historical data: he has modern data from a variety of personality tests and measures. The data he uses for this purpose were all collected before 1981: they are from studies reviewed in a 1983 book by the Swiss psychologists Cecile Ernst and Jules Angst (that's right, Ernst and Angst -- I'm not making this up). Ernst & Angst concluded that most of the studies they reviewed were worthless because the researchers had failed to control for family size and/or socioeconomic class (variables that are themselves correlated). They threw out the worthless studies, looked closely at the ones that remained, and concluded that birth order was a crock. "This may signify," they said, "that most of our opinions in the field of dynamic psychology may have to be revised."
Sulloway reexamined the same studies that Ernst & Angst reviewed -- the ones that used the proper controls -- and came to different conclusions. There are a number of problems, however, with his reexamination; I discuss them in detail in Appendix 1 of The Nurture Assumption (due out in September). For example, how many studies did Sulloway include in his reanalysis? Five times in his book Born to Rebel, and three times in his Edge interview, he gives the number of properly controlled studies as 196, but I spent days combing through Ernst & Angst's book and found nowhere near that number. The explanation of this discrepancy is contained in a note underneath a table in Born to Rebel: "Each reported finding constitutes a `study.'" Thus, if a researcher reported that the firstborns in a particular sample of subjects were more conventional, conscientious, assertive, and neurotic than the laterborns, Sulloway's definition allowed him to count these four findings as four "studies." Only by counting some studies more than once could Sulloway have obtained his total of 196. Although multiple findings generated from the same sample of subjects are not statistically independent, Sulloway nonetheless tested his data with a statistic based on the assumption that each outcome is independent.
"Unfortunately," Sulloway says in his Edge interview, "most psychologists -- to this day -- do not appreciate the issue of statistical power." He is objecting to attempts to test his claims with samples of only 200 to 400 subjects. Well, if birth order effects were as big and important as Sulloway implies, 200 to 400 subjects should be plenty to demonstrate them. In any case, he has given the impression that bigger studies are more likely than smaller ones to turn up significant birth order effects, which is what you'd expect if birth order effects were real but small. Just the opposite is true, however. Of the research reviewed by Ernst & Angst, only 19 percent of the findings from the largest studies (more than 400 subjects) were favorable to Sulloway's theory, versus 38 percent of the findings from the smallest ones (fewer than 200). Sulloway calls his reanalysis a "meta-analysis," but that term is usually used to describe a procedure that takes into account the size of the included studies and the magnitude of their effects. Neither sample size nor effect size was taken into account in Sulloway's analysis.
The largest study I know of on birth order is the one carried out by Ernst and Angst themselves. Not content to survey the work of others, they decided to check up on their conclusions by running a massive study of their own: 7,582 college-age residents of Zurich served as subjects. Ernst & Angst used all the proper controls and measured (with a self-report questionnaire) twelve different aspects of personality, including Sulloway's favorite, openness. They found no significant birth order effects at all among subjects from two-child families -- no differences in personality between the firstborn and the secondborn. Among subjects from larger families there was one significant effect: the lastborn tested slightly lower in masculinity. This study was reported in the same 1983 book that produced the data for Sulloway's reanalysis, but he does not mention it either in Born to Rebel or in his interview on Edge.
Perhaps he discounted it because it used the self-report method. Studies that use family members -- parents or siblings -- to assess the subjects' personality are far more likely to produce findings favorable to Sulloway's theory. Several such studies were included in Ernst & Angst's survey and most of them yielded multiple findings. But are the findings valid? Ernst & Angst didn't think so. When you ask people to assess the personality of their children or siblings, what you get is a description of how the subjects behave at home -- how they behave with their parents and siblings. This doesn't tell you much about how they behave at other times and in other places. Parents' descriptions of their kids agree poorly with teachers' judgments. (I imagine that teachers must get tired of hearing parents ask, "Are you sure you're talking about MY kid?") A method Sulloway advocates in his Edge interview is to have subjects compare themselves to their siblings, but what that would give you is a picture of how the siblings behave vis-a-vis each other -- how they behave when they're together, because they don't know how their sibling behaves when they're apart. I have no doubt that such a procedure would generate birth order effects.
In response to John Brockman's question about children without siblings, Sulloway hypothesizes that "only children ought to be intermediate on many personality traits" because "they are not being pushed by a younger sibling into being particularly conscientious or aggressive; and they are not being pushed by an elder sibling into being particularly daring or unconventional." But he also says that only children ought to be more variable because they "are free to occupy any niche." What Sulloway is trying to explain here is the embarrassing fact -- embarrassing not just to him but to all believers in the nurture assumption -- that only children do not differ in any systematic way from children with siblings. These children have missed out on the experiences that play such an important role in Sulloway's theory: they haven't had to compete with their siblings for parental attention, and they haven't had to learn how to get along (or not get along) with a bossy older sister or a pesky younger brother. And yet their personalities are indistinguishable from those of children with siblings.
Occasionally a study does turn up a difference between only children and children with siblings, or between firstborns and laterborns, or between first- and lastborns and middle children. Such results are a testimony to the persistence with which researchers look for them and their refusal to take no for an answer. The fun part comes in thinking up an explanation for each significant effect that is found, because each study that produces a publishable result tends to produce a different one. Sulloway mentions, for example, a study that found that middle children were less likely than first- or lastborns to identify themselves with a family label, presumably because they were more closely identified with their peers. Sulloway's explanation is that "middle children are at a disadvantage -- they don't have the benefit of being first, which leads to greater parental investment because firstborns are closer to the age of reproduction. The lastborn has the benefit of being the last child the parents are going to have, so parents will tend to invest heavily in this child so that it will not die in childhood." Ernst & Angst had something to say about this kind of post-hoc reasoning and I think it's worth quoting here. The italics are theirs.
Birth order research seems very simple, since position in a sibship and sibship size are easily defined. The computer is fed some ordinal numbers, and then it is easy to find a plausible post hoc explanation for any significant difference in the related variables. If, for example, lastborn children report more anxiety than other birth ranks, it is because for many years they were the weakest in the family. If firstborns are found to be the most timid, it is because of incoherent treatment by an inexperienced mother. If, on the other hand, middle children show the greatest anxiety, it is because they have been neglected by their parents, being neither the first- nor the lastborn. With some imagination it is even possible to find explanations for greatest anxiety in a second girl of four, and so on, ad infinitum. "This kind of research is a sheer waste of time and money."
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