followed by the add-back of hypothesized mediators. Such controlled removal and add-back can be achieved at the genetic, protein, physiological, behavioral, or social-environment level. Animal models also allow for invasive examination of organ, tissue, and region-specific mechanisms at the physiological, cellular, and molecular levels. Also animals with short reproductive cycles and life spans provide an invaluable tool for conducting developmental and life-span studies, and animal models enable the conduct of breeding experiments and genetic manipulation that facilitate the elucidation of inherited traits and genetic effects.

Animal research can serve as models of gene-environment interactions and diseases identified in humans. In the case of social control of disease processes, the choice of species to be studied depends on the level of social interactions that needs to be examined. For example, rodent models can demonstrate how differences in social status, population density, or early experiences interact with genetic makeup to affect susceptibility to disease (e.g., examine effects of social factors in knockout or knockin animals [or inbred strains] that differ in susceptibility to infection, cancer, autoimmunity). The advantages of rodent models include significant control over genetic, physiological, behavioral, and social factors and relatively short reproductive, developmental, and life cycles. They are amenable to studying a variety of important psychosocial variables, including social isolation, social relationships, attachment, parenting, temperament, and motivational states.

However, nonhuman primate models, which offer limited control over genetic factors and have a longer life span, may be best suited to examine the consequences of more complex social factors, such as those involving cooperation or trust. For example, after bouts of aggression, nonhuman primates demonstrate reconciliatory behavior that is thought to be important for maintaining cooperative social hierarchies (de Waal, 2000). Some aspects of human behavior (e.g., optimism, hope, guilt) may be studied in animals only when the investigator can demonstrate a robust animal model with multiple behavioral paradigms as well as shared neural mechanisms.

In addition, animal models developed for traditional biomedical research are also powerful models for studying the psychosocial modulation of known mechanisms of specific human diseases. There are many animal species, strains, and transgenic models developed through biomedical science, that have been well characterized in terms of the genetic, molecular,