The term intelligence has historically referred to individual differences in the assessment of school-related abilities. Recently, however, the term has been broadened to include individual differences in a number of different domains. Academic intelligence, for example, is not correlated with measures of practical intelligence, defined as the ability to solve problems that arise in natural, nonschool settings (Wagner & Sternberg, 1986). Broadening the definition even further, Gardner (1983) suggested seven different types of intelligence, including traditional academic domain, (e.g., logical-mathematical intelligence), bodily kinesthetic intelligence, musical intelligence, and social intelligence (both intrapersonal and interpersonal). The domain of intelligence that is the focus of this chapter is infants' social cognitions regarding self and other.

How do infants construct understandings of themselves and others? A long theoretical tradition suggests that infants create the self by differentiating self from nonself (Bretherton, 1985; Mahler, Pine, & Bergman, 1975; Stern, 1985). In a review of current theories of self-development, Brownell and Kopp (1991) suggest that with development, infants establish boundaries that define the self compared to others and objects. The core function of self is “to define, locate, demarcate the world from a consistent perspective by organizing, integrating and representing experiences from that vantage point” (p. 288). By creating boundaries between self and other, the construction of the self is hypothesized to parallel construction of the other (Baldwin, 1899).

The process of differentiating self from other may follow a different developmental course in twins compared to singletons.