10) Emde, R.N., Robinson, J.L., Corley, R.P., Nikkari, D., & Zahn-Waxler, C. (2001). Reactions to Restraint and Anger-related expressions during the second year. In R. Emde & J. Hewitt (Eds.), Infancy to Early Childhood: Genetic and Environmental Influences on Developmental Change (pp 127-140). Oxford University Press: New York, New York.

351 twin pairs were assessed during home and laboratory visits at ages 14,20 and 24 months. During these visits the twins experienced several “restraint” activities designed to measure anger-related expressions. A maternal interview also assessed anger-related expressions in the twins. Researchers found that there was a striking lack of observable correlation across measures. In this study, there was no evidence to support a single construct of anger across measures. The researchers suggest that the anger response may become integrated at a later age than that of the subjects. More research is needed to determine the validity of this hypothesis.