The description of the facts fits the diagnostic criteria in all ways but one. That one is the prevalence of the disorder. The APA considers that phobias must be rare, affecting only a small proportion of the population, so that the victim recognizes that his or her fear is unusual. This criterion serves to distinguish simple phobias from other disorders in which the victim believes that his or her condition is the universal state of mankind. A victim of paranoia may believe that he or she is the only person targeted by a particular evil entity (or believe that all persons of some class are so targeted), but he or she believes completely in the existence and danger of that evil entity. On the contrary, the person with the typical simple phobia, say of air travel, as in one of the examples given, has to recognize that most people do not fear air travel because air travel is the most frequently used form of long-distance travel, in which typical people complain of missed connections, lost baggage, and inconvenient travelling hours, but do not complain of being afraid. The distinction between a condition in which the victim recognizes the atypical nature of his or her fear and one in which the victim believes absolutely in the danger of the object feared is vital to making a correct diagnosis and undertaking the proper treatment.

However, the APA probably has never had to consider that a particular phobia is held by the great majority of the population. There is no theoretical reason why a condition that fits all the other diagnostic criteria for a phobia should not affect the majority of the population. However, if the great majority of the population suffer from a particular phobia, there is no means by which any victim can conclude that their fear is atypical. Under this condition, it is in fact typical. The only means by which a person who suffers from this condition can determine whether it is a phobia is by knowing whether or not the facts agree with reality, and it is recognized that knowing the facts does not cure phobias. That is, the fact that the person who fears air travel recognizes that air travel is not very dangerous (Diagnostic Criterion F), because most people do not fear air travel, does not cure the fear. The persistence despite accurate knowledge is what makes it a phobia. Only successful experience of the conditions of air travel (starting by visiting planes that will not take off) has been found to cure the disorder.