forts to verify the model by studying prisoners found guilty of either violent or nonviolent crimes suggest that the model shows behavioral differences in these two subpopulations (D. Cherek, University of Texas, personal communication, 1995). Continued research is needed to develop models of aggression in animals and humans.

Vulnerability to Drug Use

Animal models of drug self-administration have identified factors that facilitate the addiction process. For example, genetic strain (e.g., George and Goldberg, 1989) and individual differences in activity level (e.g., Piazza et al., 1993) can predict vulnerability to repetitive drug use. Environmental conditions such as lack of alternative reinforcers, restricted access to food (e.g., Carroll, 1995), and drug history (e.g., Horger et al., 1991) can accelerate the onset of drug self-administration. Behavioral studies to delineate those facilitating variables in animals may lead to data-based programs for targeting high-risk human populations, making education and prevention efforts more focused and presumably more effective.

Research in the area of etiology has focused on risk factors, with the underlying assumption that some drug use is pathological. However, one of the messages from animal research using the self-administration model is that drugs easily serve as reinforcers and that conditions do not need to be pathological for drugs to be repeatedly self-administered by all animals. Research in the area of neurobiology is beginning to demonstrate that drug-taking behavior is controlled by brain mechanisms developed through evolution to ensure the reinforcing effects of biologically essential activities of eating, drinking, and copulating. The implication of these research findings is that, were it not for countervailing influences, drug use would be the norm, not an aberration. That inference may be somewhat strong, since there are individual differences in those brain systems that contribute to vulnerability, but it points to a research effort in prevention that takes into account the biological foundations against which these efforts are made.

For example, a variety of environmental risk factors can affect responsivity to drugs, including personality, family, and peer influences. Etiological research has identified issues of interest in these areas, including questions related to risk taking, impulsivity, and deviance (see Chapter 5). Those areas have received attention from behavioral researchers in other contexts, and it should be possible to adapt existing models or to develop new ones with direct relevance to drug abuse and dependence. One etiological hypothesis that might be tested with these models is drug effect expectancy. It has been hypothesized that individuals learn about

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