Author: Kaul Bhavika
Date: July 2007
A study of nearly 250,000 military conscripts found a favorable relationship between low birth order and high IQ in families with multiple children. The difference between the eldest child and the closest sibling is almost 3 IQ points, according to the study conducted by Norwegian epidemiologists Dr. Petter Kristensen and Dr. Tor Bjerkedal.
Using military records, the study analyzed data on birth order, general health and IQ scores of 241,310 men born between 1967 and 1976. After adjusting for parental education level, maternal age at birth and family size, the Norwegian researchers found that the eldest child scored an average of 103.2 on IQ tests while the second child scored an average of 100.3- a difference of almost 3 percent points, which is statistically significant.
Furthermore, researchers examined scores of those men who became the oldest after a sibling passed away. Their scores were on average the same as those of biological first-born men suggesting that social rank in family plays a greater role in intelligence than biological factors alone.
According to Dr. Kristensen, "this is quite firm evidence that the biological explanation is not true."
Although the study was done in men, researchers say that because sex has little influence on IQ scores, the results would almost certainly apply for women as well.
The relationship between birth order and intelligence is hard to analyze and some critics are still skeptical about the results of this study. Joseph Lee Rodgers, a psychologist at the University of Oklahoma, said the new analysis was not conclusive.
"Past research included hundreds of reported birth order effects that were not legitimate," Dr. Rodgers said. "I'm not sure whether the patterns in the study are real or not."
The results were reported in Science and although still under scrutiny, could lead to more intensive studies on the relationship between birth order and intelligence.
- By Bhavika Kaul.