We examined broad dimensions of children's personalities (total n = 1056; age = 3.5 to 12 years) based on observers' perceptions following a few hours of structured interaction. Siblings' behaviours during a 2-hour cognitive assessment in the home were rated separately by two different observers. Exploratory and confirmatory factor analyses clearly revealed a two-factor solution in three different samples. There was correspondence between parent-rated temperament and observer-rated factors. Cross-sectional analyses indicated lower Plasticity among older children and higher Stability among older children. Sex differences were negligible. Plasticity and Stability were correlated in the .2 to .3 range. Most of the sibling similarity in the Plasticity was due to additive genetic influences, whereas most sibling similarity in Stability was attributable to shared environmental influences. The findings implicate a biometric factor structure to childhood personality that fits well with emerging biosocial theories of personality development. Copyright © 2012 John Wiley & Sons, Ltd.