Who's the Smart Sibling? New studies weigh in on the age-old controversy over whether firstborns are more intelligent than seconds.

Byline: Mary Carmichael (With Jesse Ellison)

Ten weeks ago, Bo Cleveland and his wife embarked on a highly unscientific experiment--they gave birth to their first child. For now, Cleveland is too exhausted to even consider having another baby, but eventually, he will. In fact, he's already planned an egalitarian strategy for raising the rest of his family. Little Arthur won't get any extra attention just because he's the firstborn, and, says his father, he probably won't be much smarter than his future siblings, either. It's the sort of thing many parents would say, but it's a bit surprising coming from Cleveland, who studies birth order and IQ at Pennsylvania State University. As he knows too well, a study published recently in the journal Science suggests that firstborns do turn out sharper than their brothers and sisters, no matter how parents try to compensate. Is Cleveland wrong? Is Arthur destined to be the smart sibling just because he had the good luck to be born first?

For decades, scientists have been squabbling over birth order like siblings fighting over a toy. Some of them say being a first-, middle- or lastborn has significant effects on intelligence. Others say that's nonsense. The spat goes back at least as far as Alfred Adler, a Freud-era psychologist who argued that firstborns had an edge. Other psychologists found his theory easy to believe--middle and youngest kids already had a bad rap, thanks to everything from primogeniture laws to the Prodigal Son. When they set out to confirm the birth-order effects Adler had predicted, they found some evidence. Dozens of studies over the next several decades showed small differences in IQ, scholastic-aptitude tests and other measures of achievement. So did "anecdata" suggesting that firstborns were more likely to win Nobel Prizes or become (ahem) prominent psychologists.

But even though the scientists were turning up birth-order patterns easily, they couldn't pin down a cause. 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. But it's hard to think that favoritism toward firstborns exists in modern society. Most of us no longer view secondborn as second best, and few parents will admit to treating their kids differently. 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 ...