37. Zahn-Waxler, C., Robinson, J. L., & Emde, R. N. (1992). The development of empathy in twins. Developmental Psychology, 28, 1038-1047.

This study examined empathy in 94 monozygotic (MZ) and 90 dizygotic (DZ) twin pairs during the second year of life. Children's reactions to simulations of distress in others were videotaped in home and laboratory settings. Some components of concern for others increased with age between 14 and 20 months for both MZ and DZ twins. Girls scored higher than boys on most of these observational measures. The different components (such as empathy, social behaviors, and mental processing) were related to each other and somewhat stable over time. There was moderate evidence for heritability of empathy. Maternal reports of social behaviors indicated both genetic and environmental influences.