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Several characteristics help distinguish MPI in the workplace from illness outbreaks due to physical causes: no laboratory or physical findings confirming an specific organic cause evidence of specific physical or psychological stressors victims are mostly women and those of lower socioeconomic status in the workplace hyperventilation-type symptoms are prominent apparent transmission by audiovisual cues rapid spread of the illness followed by rapid remission of symptoms, unless symptoms are fixed by physicians and litigation benign morbidity If a physical or chemical cause of the illness is not obvious and if episodes recur, the illness remains a mystery and the workplace becomes a breeding ground for anxiety, confusion, fear, and rumor. Economic pressures to resume normal operations are counteracted by fears that an environmental contaminant still exists. Once all physical explanations have been ruled out, investigators may turn to psychological explanations and focus on mass hysteria, but that explanation is primarily based on the absence of physical evidence rather than the presence of agreed-upon psychosocial conditions. Some reports suggest that the diagnosis of MPI is really an excuse for not conducting an adequate evaluation of low-level environmental contaminants. The disability in some workers is prolonged and points to non-work-related factors that may be contributing. Were it not for the unverifiable physical complaints and the workers' insistence that the symptoms are the result of physical disease and their concern about the complaints and their reactions to dismissal of their complaints, we would not consider these patients to have a mental disorder. These individuals are reinforced in their beliefs by some physicians and attorneys, and resultant litigation tends to fix their symptoms. In fact, a "conspiracy theory" evolves when there are enough convinced people--and enough people trying to dissuade them. Group phenomena reinforce their beliefs and make renouncement tantamount to betrayal. The mass psychogenic illness or psychiatric epidemic that occurs in the workplace offers an opportunity not only to study the elements of a biopsychosocial system, but also offers opportunities for medical anthropological studies that relate the causes and courses of specific episodes of MPI to the cultures of the individuals who develop symptoms and the cultures of the area in which the epidemic occurs.