Top of the class: first-borns are, on average, 2.3 IQ points more intelligent than their younger brothers and sisters.CorbisEldest siblings are, on average, 2.3 IQ points more intelligent than their younger brothers and sisters, says a study of Norweigan kids. And it's not necessarily being born first that makes the difference — it's being raised as the eldest child.
It has been proposed for some time that, on average across a population, first-borns are more intelligent than their younger brethren. There are more first-born sons in prominent positions than might be expected, for example. And some studies have shown a link between birth order and intelligence: the later born, the less smart the child.
But the reasons behind this trend, and even whether it's real, have been hotly debated. Families with low-intelligence children tend to be large (perhaps a big brood leaves little time for helping with homework), so the observation that sixth-born children aren't very smart, for example, could just be a side effect of this, critics have said.
Petter Kristensen, from the University of Oslo, and Tor Bjerkedal from the Norwegian Armed Forces Medical Services in Oslo looked at data gathered from 241,310 Norwegian kids, all aged 18 or 19 years old at the time of intelligence testing.
The mean IQ of first-born kids was just over 103, second-borns just over 100, and third-borns about 99, they found. But if a child's elder sibling had died, leaving him or her to be raised as first-born, their IQ lept up to match the top scores of 103. Likewise if both of two elder siblings had passed away, these third-born children had IQs matching that of first-borns, they report in Science1.
And it isn't just that big families have less smart kids, and small families have bright ones. In a separate analysis, to be published in Intelligence, the researchers show that the trend holds true for distinct pairs of siblings in their study group — even within single families, older siblings are on average smarter. "There can be no confounds in this type of study, and so the theory of spurious associations has effectively been refuted in one fell swoop," says Frank Sulloway, an expert on birth order and intelligence from the University of California, Berkeley.
Older and wiser
What causes the difference? The fact that it's down to social upbringing rather than biological birth order leads Kristensen to think it's because of factors such as parental attention to older siblings, or time that the elders spend tutoring younger sisters and brothers.
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Kristensen, a second-born child himself, admits that he did not believe the effect was real at first. "It was intuitively wrong to me that an eldest child should have an advantage," he says, especially since studies have shown that second- and third-born children often have better health than first-born kids.
The work doesn't necessarily show that younger siblings suffer from their lower IQ, Sulloway adds. "There is considerable evidence that first-borns and later-borns are good at different things," he says, citing Charles Darwin as an example. Darwin was the fifth of six children, and didn't fare very well in some of his classes at Cambridge. But Darwin was, famously, described by his uncle Josiah Wedgewood as being a man with "enlarged curiosity". "If offered the choice of having 2.3 more IQ points or Darwin's fortunate attribute of 'enlarged curiosity', I would unhesitatingly prefer the latter," says Sulloway.
Katharine Sanderson is a reporter for Nature in London, and is the second-born of two sisters.
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