The importance of friend influence as a determinant of adolescent behavior has primarily been inferred from research that has repeatedly demonstrated the behavior of friends to be similar. Homogeneity among peers, however, could also be due to a selection process whereby adolescents choose and keep friends whose behavior is similar to their own. Most previous studies have used cross-sectional designs that cannot delineate the source of peer homogeneity; this study employed sociometric data from longitudinal research on adolescent smoking and drinking to separate and examine the selection and influence processes. Although the findings indicate some support for the friend influence model, the acquisition hypothesis from the selection model accounts for substantially more of the adolescent-friend relationship. The implications of these results for past and future research concerning the role of peers in adolescence are d i d .

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The principles that behavior is strongly influenced by the behavior of friends, and that people select each other to be friends on the basis of common characteristics, are based in large part on the many studies of adolescents that found the behavior of friends to be similar. In most of these studies, subjects were asked to describe, the behavior of their friends, and this information was then related to subjects' reports of their own behavior. The research on adolescent smoking and drinking described in this paper departs from that tradition by including friend reports of friend behavior so that results based on the different sources for measuring friend behavior can be compared. Subject report of friend behavior is a stronger correlate, of adolescent behavior than friend report of friend behavior, but the two sources for measuring friend behavior produce the same correlations with subject behavior when using longitudinal designs and controlling for prior subject behavior. The findings are discussed in the context of theoretical and methodological considerations of peer influence and selection.

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A classic social psychological model is that external variables influence behavior through their impact on subjective expected utility, the extent to which more good or harm is expected from behavior. The purpose of this research was to determine whether subjective expected utility is a major intervening variable that links external variables and the onset of alcohol drinking and cigarette smoking. The data are from two longitudinal studies of adolescents. The findings suggest that subjective expected utility does not account for the relationship between external variables and behavior.A classic social psychological model is that external variables influence behavior through their impact on subjective expected utility. Subjective expected utility is the degree to which favorable or unfavorable consequences are expected from a behavior. External variables are any variables considered to influence behavior indirectly through their impact on intervening variables.Although many studies have related external variables and subjective expected utility to behavior, none have directly determined whether subjective expected utility serves as an intervening variable. This is illustrated by the many studies of adolescent alcohol drinking and cigarette smoking in which demographic, social, and personality variables have been correlated with those behaviors, and the paths through which these external variables are related to behavior have been identified by conjecture rather than by analyses designed to identify intervening variables directly. It is this void in direct empirical scrutiny to identify intervening variables that is our focus.Subjective expected utility is a central variable in many modern theories of behavior (Edwards