A study published last Friday in the journal Science reopens the heated debate over environmental (as opposed to genetic) effects on intelligence. An enormous survey of sibling IQ scores in Norway found that firstborn sons had IQ scores about three points higher than second-born sons, and four points higher than the third-born (the study looked only at men, because it was based on IQ tests given to newly-drafted Norwegian soldiers in the 1960′s and 70′s.)
At first glance, this birth order effect would seem to suggest a biological cause—having to do perhaps with the higher levels of immune antibodies in the womb after a first pregnancy. But the study also looked at second-born siblings whose older brothers died in infancy, and found that in terms of IQ scores and relation to younger siblings, they belonged with the firstborns, not the second-borns. In other words, the cause seems more likely to have to do with how parents (or others) treat the oldest brother. (For Joseph Epstein’s meditations on birth-order theory, read his 1997 article O, Brother.)
But as large-scale as this study is (it examined almost a quarter million men), it still acts to highlight just how little we understand about intelligence and its relation to genetics and environment, and how prone we are to over-read and misread statistical data on intelligence.