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From Slate.com:

Eldest children have IQs three points higher than their siblings, on average, in a huge sample of young Norwegian men. The study shows this difference is due to family dynamics, not “prenatal gestational factors,” since “second- and third-born men who became the eldest in their families due to the death of one or two older siblings … had IQs close to that of firstborns.” Theories: 1) Firstborns get more parental attention. 2) They’re presented with higher expectations. 3) They benefit from teaching younger siblings. 4) They strive to achieve, so their siblings seek a less achievement-oriented identity. Critiques: 1) The difference is just an average; when sibling IQ scores differ, there’s a 43 percent chance the younger kid scores higher. 2) Smaller U.S. studies differ from the Norwegian one. 3) Previously asserted birth-order effects have been debunked. 4) Younger siblings are less likely to be smart but more likely to be brilliant. 5) Younger siblings may have lower IQs, but they’re more interesting. (To debate IQ and birth order, click here.)