Many efforts have been made to provide either a theoretical basis or an integrated framework for examining influences on smoking initiation. As a step toward an integrated approach, Petraitis and colleagues (1995) suggested that factors affecting tobacco use can be classified along two dimensions-type of influence and level of influence. These authors suggested that three distinct types of influence underlie existing theories of tobacco use-social, cultural, and personal. Social influences include the characteristics, beliefs, attitudes, and behaviors of the persons who make up the more intimate support system of adolescents, such as family and friends. Cultural influences include the practices and norms of the broader social environment of adolescents, such as the community, neighborhood, and school. Personal influences include individual biological characteristics, personality traits, affective states, and behavioral skills. For each type of influence, three levels of influence-ultimate, distal, and proximal-have been defined by work in evolutionary biology (Alcock 1989), cognitive science (Massaro 1991), and personality theory (Marshall 1991). McKinlay and Marceau (2000a,b) have emphasized the importance of a broad new integration of approaches and multilevel explanations. The levels of influence affect the nature and strength of the type of influence. Ultimate influences are broad, exogenous factors that gradually direct persons toward a behavior but are not strongly predictive. Distal influences are intermediate or indirect factors that may be more predictive. Proximal influences, which are the most immediate precursors of a behavior, are most predictive. The study of social, cultural, and personal domains among adolescents and the various levels of influence has undergone considerable theoretical development. This review of smoking initiation examined more than 100 studies in which tobacco use was an outcome variable. Selected characteristics and major gender-specific findings of the longitudinal studies are shown in Table 4.1.