A large Norwegian study suggests that social position in the family and not biological birth order as such is significantly linked to IQ. Thus a child who is the first born (or is treated as the first born if say their older sibling dies) is more likely to develop a higher IQ compared with his or her younger siblings.

The study is published in the journal Science.

Other studies have found links between low birth order and higher intelligence as measured by IQ, but the reason has not been very clear; is it biological or is it social?

For instance some theories have suggested it is social, that the stimulation and attention received by the first born before the siblings come along give the earlier born child’s intellectual development a boost, especially when you add to this their social position in the family, for example as mentors and teachers of the younger siblings.

Other theories have suggested that higher prevalence of maternal antibody attack in the later pregnancies which affects fetal brain development, and other biologically unfavourable conditions means the brains of firstborn children have a better chance of cognitive development and learning. However, until now this was just speculation.

Petter Kristensen from the National Institute of Occupational Health, Oslo, Norway, and epidemiologists from the University of Oslo and the Norwegian Armed Forces, said their findings support the notion that it is family interaction and stimulation of low birth order children and not biological factors that causes them to have higher IQ than their siblings. They did this by focusing on those children who had different social and biological ranks in the family.

For instance, a second born whose older sibling had died would have a social rank equivalent to a first born, but their biological rank would still be that of a second born. So if the biological hypothesis was right, second born children who were raised socially in the same way as the eldest, would have IQ scores equal to those of other second born children. This is not what Kristensen and colleagues found.

The researchers had access to data on birth order, status of earlier born siblings (whether they had died early in infancy for example), and IQ scores for nearly 250,000 male 18 and 19 year old Norwegian military conscripts who were drafted between 1985 and 2004.

They found, using linear regression analysis, and taking into account a number of factors that might for example affect IQ in families with adverse reproductive histories, that men who were first in social or birth order had, on average an IQ about 2.3 points higher than those who were second in social or birth order. This pattern continued in the sense that second born men had higher IQs than the third born, and so on.

But when Kristensen and his team removed the effect of social order, they found that the effect of birth order was statistically non-significant. This meant that it was social order and not birth order that gave the “eldest” children the higher IQ points.

Some critics have said that these associations are “artefacts”, or spurious statistical phenomena. But in a separate study to be published in the journal Intelligence, Frank Sulloway of the University of California, in Berkeley said he and his colleagues showed these associations are true even when you look at pairs of siblings. Even in the same family, the older children on average have a higher IQ. Sulloway said that their study, using paired siblings, puts paid to the notion that the link between birth order or social order and intelligence is just spurious.

Kristensen, who is a second born himself, said these results went against his intuition, because other studies show that first borns tend to suffer poorer health. Also parents should not be unduly concerned about these results. Having high IQ and knowing how to use it are different attributes. A child might score a few points lower in their IQ but have other assets such as curiosity, imagination and what is increasingly being called “emotional intelligence” that helps them use their IQ more effectively.

Sulloway refers to Charles Darwin, who was child number 5 out of 6. He didn’t do so well when at Cambridge University, but his curiosity was insatiable, and that drove him to write his famous work about evolution. Sulloway said that if he had the choice between 2.3 more IQ points and Darwin’s “enlarged curiosity”, he would have no problem choosing which one.

If there is a message to parents here, then perhaps it is this. If you have several children, then spending some one-to-one time with each one is probably a good thing to do but if you can’t manage it, don’t lose sleep over it. On the whole, a happy parent shared is probably preferable to a stressed and anxious parent all to oneself.

“Explaining the Relation Between Birth Order and Intelligence.”
Petter Kristensen and Tor Bjerkedal.
Science 22 June 2007.
Vol. 316. no. 5832, p. 1717.
DOI: 10.1126/science.1141493.

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Written by: Catharine Paddock
Writer: Medical News Today