The obvious culprits on the nurture side are parents. But it’s hard to think that favoritism toward firstborns exists in modern society. In surveys, they generally say they give their children equal attention. Kids concur, reporting that they feel they’re treated fairly.

Maybe, then, 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 big, fat, methodological flaw. The Norwegian study is an example, says Cleveland: “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 big thing: 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 two 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 are there — within families, birth order really does seem linked to brain power. Even the critics have to soften their positions a little. The Intelligence study “must be taken very seriously,” says Rodgers.
-MARY CARMICHAEL (Newsweek)