Children raised as the eldest showed slightly higher IQs than their younger siblings, first-born children possess IQs that are 2.3 points higher, on average, than their younger siblings, revealed by Norwegian researchers.
The study, published in the journal Science, suggests that it’s how the kids are raised, not their birth order, that counts.
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.