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General cognitive ability (g) can be derived from highly replicable assessments of processing speed and verbal, memory, and spatial abilities. To calculate the genetic heritability of g, researchers analyzed test differences in 11,000 monozygotic (MZ) or dizygotic (DZ) twins who had undergone comprehensive evaluations. Standard statistical methods for twin studies were used to estimate the effect of additive genetic (twice the MZ–DZ difference), shared environmental (MZ-additive genetic) and unique environmental (nonshared MZ variance) factors. Analyses were conducted across the six studies examined and according to age group (mean ages, 9, 12, and 17 years).
In all age groups, the best-fitting statistical model included additive genetic, sh…