The difference in I.Q. was small—an average of three points higher in the oldest child than in the next sibling—but the researchers consider it significant. Why should that be so? Perhaps, one theory went, the mother’s body was somehow attacking the later offspring in utero. Maternal antibody levels do increase with each successive pregnancy. But there’s no evidence that this leads to differences in intelligence, and the new study in Science, based on records from nearly a quarter of a million young Norwegian men, strikes down the antibody hypothesis. It looks at kids who are the eldest by accident—those whose older siblings die in infancy—as well as those who are true firstborns. Both groups rack up the same high scores on IQ tests. Whatever is lowering the latterborns’ scores, it isn’t prenatal biology, since being raised as the firstborn, not actually being the firstborn, is what counts. The obvious culprits on the nurture side are parents. While it’s hard to think that favoritism toward firstborns exists in modern society—and most parents say they give their children equal attention—logic suggests that firstborns benefit from having their parents’ undivided attention in the first, formative years of their lives.
But maybe the problem with latterborns isn’t nature or nurture—maybe there simply isn’t a problem. Not all the research shows a difference in intelligence. A pivotal 2000 study by Joe Rodgers, now a professor emeritus at the University of Oklahoma, found no link between birth order and smarts. And an earlier study of American families found that the youngest kids, not the oldest, did best in school. From that work, says psychologist Judith Rich Harris, a prominent critic of birth-order patterns, it’s clear that “the impression that the firstborn is more often the academic achiever is false.”
Meanwhile, many of the studies showing a birth-order pattern in IQ have a major methodological flaw. The Norwegian Science study is an example, says Bo Cleveland, a psychologist who studies birth order and IQ at Pennsylvania State University: “It’s comparing Bill, the first child in one family, to Bob, the second child in another family.” That would be fine if all families were identical, but of course, they aren’t. The study controls for variables such as parental education and family size. But Rodgers, the Oklahoma professor, notes that there are “hundreds” of other factors in play, and because it’s so hard to discount all of them, he’s “not sure whether the patterns in the Science article are real.”
No one is more sensitive to that criticism than the Norwegian scientists. In fact, they already have an answer ready in the form of a second paper. Soon to be published in the journal Intelligence, it’s similar to the Science study except for one key factor: instead of comparing Bill to Bob, it compares Bill to his younger brothers Barry and Barney. The same birth-order pattern shows up: the firstborns, on average, score about three points higher than their secondborn brothers, and hapless thirdborns do even worse. “The purpose of the two papers was exactly the same,” says Petter Kristensen of Norway’s National Institute of Occupational Health, who led both new studies. “But this second one is much more comprehensive, and in a sense it’s better than the Science paper.” The data is there—within families, birth order really does seem linked to brainpower. Even the critics have to soften their positions a little. The Intelligence study “must be taken very seriously,” says Rodgers.
No one, not even Kristensen, thinks the debate is over. For one thing, there’s still that argument about what’s causing birth-order effects. It’s possible, says Berkeley researcher Frank Sulloway, that trying to treat kids in an even-handed way in fact results in inequity. Well-meaning parents may end up shortchanging middleborns because there’s one thing they can’t equalize: at no point in the middle child’s life does he get to be the only kid in the house. Alternately, says Sulloway, there’s the theory he has his money on, the “family niche hypothesis.” Older kids, whether out of desire or necessity, are often called on to be “assistant parents,” he notes. Getting that early taste of power and responsibility may prime them for achievement later on. “If they think ‘oh, I’m supposed to be more intelligent so I’d better do my homework,’ it doesn’t matter if they actually are more intelligent,” says Sulloway. “It becomes a self-fulfilling prophecy.” If the firstborns’ homework involves reading Science and Intelligence, there’ll be no stopping them now.
