Do different measures tap the same genetic influences? A multi-method study of activity level in young twins.

Activity level (AL) is a highly salient feature of child behaviour that has been linked to developmental outcome. Twin studies of parent-rated, observer-rated and mechanically assessed AL in childhood find that AL is genetically influenced. Few studies, however, consider whether different methods of assessing AL have a shared genetic etiology. Those that do, confound methods and situations. The present study examined whether actigraph and rater-based (parent, observer) measures of AL tap the same genetic influences in a sample of 312 2-year-old twin pairs. Methods were studied within the same situation, thereby controlling for situational influences on AL. The genetic correlation between actigraph and parent-rated AL in the home was .38, indicating modest genetic overlap between the two methods. In contrast, the correlation of genetic effects on actigraph and observer-rated AL in the laboratory was .95, indicating that both laboratory-based measures of AL are influenced by the same genetic effects.