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The behavioral immune system is a phrase coined by the psychological scientist Mark Schaller to refer to a suite of psychological mechanisms that allow individual organisms to detect the potential presence of parasites in their immediate environment, and to engage in behaviors that prevent contact with those objects and individuals.

These mechanisms include sensory processes through which cues connoting the presence of parasitic infections are peceived (e.g., the smell of a foul odor, the sight of pox or pustules), as well as stimulus-response systems through which these sensory cues trigger a cascade of aversive affective, cognitive, and behavioral reactions (e.g., arousal of disgust, automatic activation of cognitions that connote the threat of disease, behavioral avoidance).

The existance of a behavioral immune system has been documented across many animal species, including humans. It is theorized that the mechanisms that comprise the behavioral immune system evolved as a crude first line of defense against disease-causing pathogens.

Implications for Human Behavior
Within the psychological sciences, there is extensive research linking the behavioral immune system to a variety of prejudices -- including prejudices against people who aren't actually diseased but are simply characterized by some sort of visual characteristics that deviate from those of a subjectively prototypical human being. The disease-avoidant processes that characterize the behavioral immune system have been shown to contribute to prejudices against obese individuals, elderly individuals, and people with physical disfigurements or disabilities. In addition, the behavioral immune system appears to contribute to xenophobia and ethnocentrism. One implication is that these prejudices tend to be exaggerated under conditions in which people feel especially vulnerable to the potential transmission of infectious diseases.

Additional lines of research on the behavioral immune system have shown that people engage in more reticent and conservative forms of behavior under conditions in which they feel more vulnerable to disease transmission. For instance, when the potential threat of disease is made salient, people tend to be less extraverted or sociable.

Implications for Human Culture
The behavioral immune system also has consequences at a cultural level of analysis. Under ecological circumstances in which parasitic diseases are more prevalent, people tend also to show more reticent and conservative forms of behavior, and cultures are defined by more conservative value systems.