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.
Perhaps unsurprisingly, coverage of the study was rampant with speculation. The New York Times imagined all manner of possible reasons for the IQ difference—none of them supported by the newly published research. The story’s most developed conjecture ran as follows:
Firstborns have their parents’ undivided attention as infants, and even if that attention is later divided evenly with a sibling or more, it means that over time they will have more cumulative adult attention, in theory enriching their vocabulary and reasoning abilities.
But there’s a small problem. This explanation is flatly contradicted by an extensive literature (actually acknowledged in the same Times story) showing that before about the age of twelve, younger siblings actually score higher on IQ tests than older. This latest study strongly suggests that disparity is reversed by the time the siblings reach early adulthood, though it offers little sense of how or why.
Inconsistencies like these tell us less about the state of intelligence research than they do about our social sensitivity regarding this subject. Our egalitarian culture is implicitly committed to a blank-slate notion of intelligence, by which we are all born with the same capacities, and what matters is the chance we are given to hone them. Research that tends to support this view, as the Norwegian study does, is held to prove it utterly (the Times quotes one of the study’s authors as saying “This is quite firm evidence that the biological explanation is not true”), while every study suggesting otherwise is cut down by a thousand counter-explanations.
From all this speculation, though, there does emerge something like a general conclusion: much of the research on intelligence in recent decades has shown that genetics sets a general range above (and below) which particular individuals will not go. But that range is reasonably broad, and where an individual falls within it is affected by parenting, by culture and community, and by nutrition and health—factors, it seems, which tend to gang up against us younger siblings. (Don’t tell my brother.)