According to a large study, the oldest children in families tend to develop higher IQ's than their siblings.
These findings could settle more than fifty years of scientific debate about the relationship between IQ and birth order.
There was, on average, a three point difference in IQ between the oldest child and the next closest sibling, a slight but significant difference.
The results made it clear that the difference was a result of family dynamics, rather than biological factors; children who became the oldest child after the early death of a sibling had the same scores, on average, as biological firstborns.
The study looked at the birth order and IQ scores of more than 240,000 men aged 18 or 19, based on their military records.
After correcting for factors that can affect scores, such as parents' education level, researchers found that the oldest children scored an average of 103.2, whereas second children averaged 100.3 and thirdborn children averaged 99. The scientists then looked at the IQ scores of more than 60,000 pairs of brothers, and found the same results.
Since gender generally has little effect on IQ scores, the results likely apply to women as well as men. The New York Times June 21, 2007
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