al., 1997; Botvin, Baker, et al., 1990; Botvin, 2004). The preventive intervention tests both the malleability of identified risk factors and the causal chain leading from these risk factors to distal outcomes (Snyder, Reid, et al., 2006). These causal chains can be tested with mediation modeling (MacKinnon, 2008), which decomposes the overall effects into those that follow hypothesized pathways and those whose pathways are not identified. A mediation model that explains most of an intervention’s impact through the hypothesized pathways confirms the underlying theoretical model of change, whereas if the hypothesized pathways contribute little explanatory power, a new theory (or better mediating measures) needs to be developed to explain an intervention’s effects.

More detailed models of etiology can be developed with analyses that examine the variations across subgroups and environments in the impact of an intervention on both mediators and distal outcomes (Kellam, Koretz, and Moscicki, 1999; Howe, Reiss, and Yuh, 2002; MacKinnon, 2008). For prevention of drug use, for example, a universal intervention that (1) builds social skills to resist the use of drugs, (2) gives feedback to young people about the true rate of peers’ drug use, and (3) enhances coping skills could well have very different effects on young people who are current drug users and those who are nonusers. Understanding such differences can lead to an elaboration of knowledge of how peer messages and media images influence initiation and escalation behavior, as well as the roles played by personal and social skills (Botvin and Griffin, 2004). Griffin, Scheier, and colleagues (2001), for example, identified psychological well-being and lower positive expectancy toward drug use as key mediators between competence skills and later substance use.

Preventive trials can also examine the causal role of a particular risk factor when it is targeted by an intervention. For example, continuing aggressive or disruptive behavior early in life is a strong antecedent to a wide range of externalizing behaviors for both boys and girls (Ensminger, Kellam, and Rubin, 1983; Harachi, Fleming, et al., 2006). While these behaviors are much less frequent for girls than for boys, the long-term risk of any problem behavior is high for both sexes (Bierman, Bruschi, et al., 2004). Nevertheless, there are important differences in the specific risks and mediation pathways (Moffitt, Caspi, et al., 2001; Ensminger, Brown, and Kellam, 1984). The long-term link between individual-level aggression in first grade and adult antisocial personality disorder has been found to be both stronger and also more malleable by the Good Behavior Game (see Box 6-8) for boys compared with girls (Kellam, Brown, et al., 2008), which points to differences in the causal role of this risk factor for boys and girls.

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