Factors contributing to individual differences of empathy were examined using behavioral genetics methodology. Data related to individual levels of empathy and parental warmth received during childhood were collected from approximately 450 pairs of twins (ages 14-33). A bivariate model analysis clarified that shared family environmental factors did not contribute to the formation of empathy. No common shared environmental factors were detected between empathy and parental warmth, either. The positive correlation between the two variables was mediated principally by genetics. The result does not support socialization theory, which holds that warm parenting nurtures children's empathy. However, the subsequent gene-environment interaction model analysis revealed that shared family environmental factors significantly affected the formation of empathy for those with high or very low parental warmth. The results imply that individuals with a strong or very weak attachment to their parents were more influenced by the shared family environment.