Because of the sparcity of research on infants' response to social events, especially different categories of people, infants between 8 and 18 months of age were introduced to five different social events: strange adult male and female, strange 4-year-old female, mother, and self. The infants' responses indicated that approach affects stimulus differentiation (in terms of fear), while age affects level of response intensity. Strangeness of the social event was not sufficient to explain the results, and ethological and cognitive theories are discussed. It is suggested that the cognitive construct of self be introduced to explain the results. It appears reasonable to assume that by eight months the self as differentiated from other is well established, and some specific categories of self, such as size, sex, or efficacy, may also be established. (Author/LH)