Genetic and extragenetic variance in motor performance.

A simple additive model of genes plus environment, based on intrapair similarities observed in 35 MZ and 35 like-sexed DZ twin pairs, was used to estimate the relative contribution of genetically controlled phenotypic variance in psychomotor individuality. Subjects practiced 50 trials on a pursuit rotor apparatus under a 20-sec/20-sec, work-rest schedule with a 30 min rest pause allowed between trials 30 and 31. Analyses of the data disclosed that a greater proportion of genetic factors, as opposed to nongenetic factors, appears to account for existing individual differences in motor performance among individuals subject to similar environmental conditions. The strength of this genetic control, however, systematically diminished throughout the course of practice obeying a monotonic trend over trials.