51. DiLalla, L. F., & Falligant, E. L. (1995). An environmental and behavior genetic perspective on behavioral inhibition in toddlers. In L. F. DiLalla & S. C. Dollinger (Eds.), The assessment of biological mechanisms across the lifespan (pp 91-117). NJ: Erlbaum Press.

In this study, twins were examined in a peer play situation. Identical twins were more similar to each other in terms of inhibited behavior than were fraternal twins. At 24 months the heritability was approximately 60%, suggesting that at this age genetic effects are more important than environmental effects. At 36 months the heritability estimate dropped to 30%. At the older age the twins were separated from each other during the play session and similarity for both fraternal and identical twins decreased substantially. The finding that both types of twins were more similar when they were in the playroom together implies that the twins positively affect each other when playing, such that one twin augments the other twin's shyness. This was even more true for identical twins than fraternal, which is why the heritability for this behavior was so much higher at the younger age.