processes presumably mediating them, and opportunities to conduct analyses across multiple levels of biological organization (e.g., neurocircuitry, cellular, genetic and epigenetic) that involve high levels of invasiveness. As noted in a recent Institute of Medicine (IOM; 2006) report, rodent models can be useful for studying a range of psychosocial environmental variables such as social relationships and many aspects of parenting although they may be limited for investigating more complex social factors such as those involving “cooperation or trust” (p. 133) which may require use of nonhuman primates. In considering ways to model the symptoms of major depression in mice, Cryan and Holmes (2005, Table 1) identified a number of possible models for most DSM-IV criteria including “indecisiveness or diminished ability to think or concentrate” where they suggested “deficits in working and spatial memory and impaired sustained attention.” However, they also concluded that “recurrent thoughts of death or suicide” or “feelings of worthlessness or excessive or inappropriate guilt” could not be modeled in mice (and presumably all other nonhuman animals). Similarly, some DSM-IV (American Psychiatric Association, 2000) criteria such as “ a persistent desire or unsuccessful efforts to cut down or control substance use” and “the substance use is continued despite
