19) Robinson, J.L., Zahn-Waxler, C., & Emde, R.N. (2001). Relationship context as a moderator of sources of individual differences in empathic development. In R. Emde & J. Hewitt (Eds.), Infancy to Early Childhood: Genetic and Environmental Influences on Developmental Change (pp 292-306). Oxford University Press: New York, New York.

Researchers examined empathic responses in twins both to their mother and to a female examiner at 14,20,24 and 36 months. They found that the twins reacted differently depending on the relationship, with attempts to help the mother occurring 2 to 3 times as often. Analysis of the data showed a strong heritability for empathy expressed towards the examiner - identical twins were more similar in their empathic responses to the examiner than were fraternal twins. Empathic response towards the mother was influenced more by shared environment than by heredity. The researchers also noted specific effects appearing at different ages, and increasing levels of empathic behaviors as the children aged.