Mass hysteria is defined as an outbreak of physical symptoms that appear to stem from a toxic or organic cause, but turn out to be the result of shared psychological stress among a group of people.

In my early reports in The New England Journal of Medicine and other scholarly publications, I described the characteristic features of these phenomena in order to assist health officials differentiate them from epidemics caused by a physical toxin.

Searching for the following features can help investigators focus on possible psychological and social factors that contribute to such outbreaks so an effective intervention can be initiated sooner rather than later:

• Absence of laboratory results and physical findings confirming a specific toxic or physical cause

• Symptoms occurring more in females than in males

• Apparent transmission of illness by sight, sound, or both

• Presence of hyperventilation or fainting

• Symptoms more common in teenagers or preteens

• Rapid spread of symptoms followed by rapid remission

• Relapses of illness in the setting of the original outbreak

• Evidence of unusual physical or psychological stress