Firstborn children score significantly higher in IQ tests than their younger siblings, according to a large study of 250,000 military draftees in Norway.
The researchers say the difference is due to social, not biological, factors, as younger siblings have higher IQs if they are raised as an eldest child following the death of an older brother or sister.
The analysis revealed that firstborn men have, on average, an IQ that is about 2.3 points higher than those who are second-born. The trend continues such that second born men have higher IQs than their third-born brothers, and so on.
A further review of about 600 families that included at least four children showed that the eldest sibling typically has an IQ that is 2.9 points higher than the fourth-born sibling.
Sulloway calculates that a 2.3 IQ score difference means that the eldest child has a 13% higher chance of having above-average intelligence than the second-born in their family.
Sulloway explains that older siblings might have a higher IQ because they act as surrogate parents from time to time, tutoring their younger siblings in certain situations.
Another possible explanation is that parents have more time and resources to invest in their firstborn child. As a result of this, the eldest child usually has the most one-on-one time with its parents, and receives greater exposure to their sophisticated vocabulary.
NewScientist.com
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