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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 and become (ahem) prominent psychologists.

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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 the utero. Maternal antibody levels do increase with each successive pregnancy. But there’s no evidence that this leads to differences in intelligence, and a study published recently in the journal Science strikes down the antibody hypothesis. The study, based on records from nearly a quarter million young Norwegians, 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.

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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 treated fairly.

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What, then, is causing birth-order effects? It’s possible, says UC Berkeley researcher Frank Sulloway, that trying to treat kids in an even-handed way in fact results in inequality. 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. Alternatively, 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 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, there’ll be no stopping them now.