Stability of genetic determination from age 2 to age 9: A longitudinal twin study

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Abstract
In an earlier study of 46 pairs of male twins, genetic determination of certain social characteristics, speech, and vocabulary IQ was assessed at age 2.5 years. In a follow‐up of 35 of these pairs at age 9, genetic and environmental factors were examined again in the same characteristics, as well as in antisocial and neurotic behavior, nonverbal ability, school achievements, height, and weight. At 9, significant genetic variance, replicated via two methods of calculation, was found in height, weight, nonverbal IQ, and antisocial behavior (teacher's rating). Shared family environmental influences were important for compliance, conscience, and antisocial behavior (parents’ ratings). Social traits were thus less under genetic influence than were physical and cognitive characteristics, and for antisocial behavior, genetic and environmental factors were both important. Stable genetic determination across both ages was found only for speech, measured by speech rate at age 2 and speech maturity at age 9.