The general definition of social influence is that health-related behavior is influenced by a person's social context. The behavioral social context can be represented by the behaviors of individual peers or family members (e.g., smoking) with whom the person interacts regularly, or by behaviors observed in a larger social environment such as the neighborhood in which a person lives. The normative social context is represented in an individual's perceptions about the acceptability of a behavior (e.g., alcohol use), derived from communications from network members, or by portrayals of behaviors in mass media such as TV or movies that the person watches. The concept of social influence is included in a number of theoretical models that have been used to predict health-related behaviors and to guide preventive interventions. However, the way in which social influence has been conceptualized and measured varies considerably across theoretical models. This ranges from concepts of more overt forms of influence, where some individuals actively exert pressure on others, to more indirect forms where normative perceptions (e.g., perceiving smoking as frequent in the population) or social perceptions (e.g., perceiving typical smokers as cool or popular) act as a form of "silent" influence. Each conceptualization of social influence has some empirical support, but researchers should consider what aspect of social influence is most relevant for the question being studied.
In this section we first consider how the concept of social influence is represented in health behavior theory. Since there have been several different conceptions of social influence used in theoretical approaches, these are discussed separately. In the second section we describe measures that have been used to index various aspects of social influence. Examples of single-item measures and multiple-item scales are given in the section and an appendix.