First-born children possess IQs that are 2.3 points higher, on average, than their younger siblings, a new study has found.
The study also suggests that it's how the kids are raised, not their birth order, that counts, according to the Norwegian researchers who authored the report, published in the journal Science.
The investigators found that children raised as the eldest showed slightly higher IQs than their younger siblings.
Even if a child had lost an older sibling and was raised as the eldest, their IQ was higher by an average of 2.3 points than their younger siblings.
Petter Kristensen, from the University of Oslo, and Tor Bjerkedal from the Norwegian Armed Forces Medical Services in Oslo examined data gathered from 241,310 Norwegian kids, all aged 18 or 19 years old at the time of intelligence testing.
They found that the mean IQ of first-born kids was just over 103, second-borns just over 100, and third-borns about 99. But if a child's elder sibling had died, leaving him or her to be raised as first-born, their IQ shot up to match the peak scores of 103. Similarly if both of two elder siblings had expired, these third-born children had IQs matching that of first-borns.
In a separate analysis, the researchers have suggested that it isn't just that big families have less smart kids, and small families have brilliant ones. The researchers showed that the tendency holds true for different 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," Nature.com quoted Frank Sulloway, an expert on birth order and intelligence from the University of California, Berkeley, as saying.
Sulloway noted that there are several theories that might explain the difference in IQ between first-born and younger siblings. Among these is one that says that more money is spent on the oldest child, and, as family size increases, less money is available for other children, leaving them with less opportunity.
Another theory holds that the first-born child gets more of the parents' attention, but Sulloway also discounts this theory. Still another explanation is that older children teach younger children, and the act of teaching raises the IQ.
But the theory which Sulloway believes is the most apt is called "niche partitioning." This theory suggests that once a role in the family is filled, others have to find roles that help them compete for attention in the family. Because older children already occupy that niche in the family, younger children have to find other roles to play, Sulloway said.
However, the work doesn't necessarily show that younger siblings suffer from their lower IQ.
"There is considerable evidence that first-borns and later-borns are good at different things," Sulloway said.
Sulloway cited Darwin's example, saying that he was the fifth of six children, and didn't fare very well in some of his classes at Cambridge. But Darwin was, distinctively, 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," Sulloway said.