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Twin studies have shown that IQ is largely inherited and that environmental factors decrease while genetic factors increase with age (JW Psychiatry Aug 17 2009). More-direct genetic effects have been challenging to demonstrate.
These investigators sought to address this problem via cognitive phenotyping and genome-wide analysis of 549,692 single nucleotide polymorphisms (SNPs) in five birth cohorts comprising 3511 healthy adults in the U.K. (discovery sample; depending on cohort, born in 1921 or 1936 or with an age range of 44–93 at enrollment); a replication cohort consisted of 670 Norwegians (age range, 18–79). For cognitive phenotyping, researchers measured both crystallized intelligence (accumulated knowledge) and fluid intelligence (abs…