Continuity and change in objectively assessed temperament: A longitudinal twin study of activity level

Because genetic influences may change across the life-span, the finding of genetic influences on activity level (AL) at one age may leave some doubt about such influences at another. Previous research exploring the importance of genetic contributions to individual differences in motor activity across age have been clouded by subjective errors that may arise when assessments of AL rely on human judgement. The present study is a longitudinal extension of an initial study demonstrating genetic influences on mechanically assessed AL in 7-month-old twins (Saudino & Eaton, 1991). The motor activity of 35-month-old twins (37 MZ pairs, 16 DZ pairs) was re-evaluated with motion recorders and parent ratings over a two-day period. Despite a lack of phenotypic stability, mechanically assessed AL continued to show a genetic influence during early childhood (RMZ = .76, RDZ = .51). Moreover, the MZ-DZ pattern for concordance on AL change (RMZ = .75 vs. RDZ = .52) was consistent with the presence of a genetic influence on developmental change.