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The observation that children’s activity level (AL) differs between novel and familiar situations is well established. What influences individual differences in how AL is different across these situations is less well understood. Drawing on animal literature, which links rats’ AL when 1st placed in a novel setting with novelty seeking phenotypes, and child temperament literature, which links AL, novelty response, and shyness, we hypothesized that shyness would be an important component of children’s AL in a novel situation. We examined this using mechanically assessed AL from 2 situations (the home and the lab) and 2 measures of shyness (1 parent-rated and 1 observer-rated) on up to 313 twin pairs (145 monozygotic and 168 dizygotic), at 2 and 3 years of age. Biometric genetic models removed from lab AL the variance shared with home AL, representing what was different in AL when the child entered the lab compared to the home. We report that almost half (43%) of the genetic component of AL in the lab was independent of AL in the home, and this unique genetic component shared genetic covariance with shyness. Shyness influences AL in a novel situation such as the lab, indicating that mechanically assessed AL represents more than global motoric activity and provides information on a child’s temperamental response to novelty.