Sources of continuity and change in activity level in early childhood.

Actigraphs and parent and observer ratings were used to explore genetic influences on continuity and change in activity level (AL) in early childhood. Over 300 pairs of twins wore actigraphs for a 48-hr period in the home and laboratory at ages 2 and 3. AL was genetically influenced at both ages with little evidence of differential heritability across age. For all measures, genetic influences contributed to phenotypic continuity. With the exception of the actigraph measure of AL in the home, new genetic effects emerged at age 3 indicating that genetic factors influence both continuity and change in AL in early childhood. Nonshared environmental influences were also a source of change in AL across the transition from infancy to early childhood.