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This chapter considers factors important to the development of understanding of self and other in young, same-sex twins. Individual differences in temperamental characteristics, language, general cognitive abilities, and environmental and genetic influences are considered in relation to self-recognizing and role-taking abilities during the later part of the second year of life. Both abilities were related to imitative language abilities and shyness. While heritable influences were present for role-taking abilities at 24 months, common environmental influences were significant for role-taking at 20 months and for self-recognizing abilities at both 20 and 24 months of age.