"Relation between birth order and IQ score. Mean IQ scores for male conscripts, first-, second-, and thirdborn in Norway to mothers with single births only and first birth from 1967 through 1976, according to birth order and number of elder siblings who died in infancy (age < 1 year). Scores are adjusted for parental education level, maternal age at birth, sibship size, birth weight, and year of conscription. Error bars show 95% confidence intervals (CIs)"
More here; Research Finds Firstborns Gain the Higher I.Q.;
In the study, Norwegian epidemiologists analyzed data on birth order, health status and I.Q. scores of 241,310 18- and 19-year-old men born from 1967 to 1976, using military records. After correcting for factors that may affect scores, including parents’ education level, maternal age at birth and family size, the researchers found that eldest children scored an average of 103.2, about 3 percent higher than second children (100.3) and 4 percent higher than thirdborns (99.0).
The difference was an average, meaning that it varied by family and showed up in most families but not all.
The scientists then looked at I.Q. scores in 63,951 pairs of brothers, and found the same results. Differences in household environments did not explain elder siblings’ higher scores.
Because sex has little effect on I.Q. scores, the results almost certainly apply to females as well, said Dr. Petter Kristensen, an epidemiologist at the University of Oslo and the lead author of the Science study. His co-author was Dr. Tor Bjerkedal, an epidemiologist at the Norwegian Armed Forces Medical Services.
To test whether the difference could be due to biological factors, the researchers examined the scores of young men who became the eldest in the household after an older sibling had died. Their scores came out the same, on average, as those of biological firstborns.
“This is quite firm evidence that the biological explanation is not true,” Dr. Kristensen said in a telephone interview.
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