44. DiLalla, L. F., Kagan, J., & Reznick, J. S. (1994). Genetic etiology of behavioral inhibition among two-year-old children. Infant Behavior and Development, 17, 401-408.

Inhibited behavior in 157 two-year-old twin pairs was studied in a situation in which two pairs of twins, unfamiliar with each other, played together in an unfamiliar room. Observers rated how long children stared, approached, interacted, or vocalized to each other, forming an index of inhibited behavior. An intraclass correlation comparison between MZ and DZ twins showed that both heritability and environmental influences were important for inhibited behavior.