|The Third Culture||
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 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.