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Infant temperament and genetics: an objective twin study of motor activity level.

Child temperament theories generally presume genetic contributions to behavioral differences, but empirical support is based largely on parent ratings of twins. These subjective ratings may be biased by parental exaggeration of dizygotic differences or of monozygotic similarities. An objective assessment of the genetic hypothesis was undertaken with motor activity level, a core dimension of most temperament theories. The activity level of 60 infant twin pairs was measured both by parent ratings and by motion recorders over a 2-day period. Data from the motion recorders showed evidence of genetic influences (RMZ = .76, RDZ = .56), as did parent ratings (RMZ = .82, RDZ = .21). The motion recorder results confirm with instrumentation a critical assumption of temperament theories and identify the presence of genetic contributors to temperamentally relevant behavioral differences in infancy.