People who have spent a lot of time in contact with animals often develop animal-induced allergies, a significant percentage of which are quite serious. In a survey of current employees in major zoos, about 30 percent had animal-induced allergies. However, a zoo employee who develops a serious animal-induced allergy is very likely to switch to some other occupation.
 The hypothesis that the incidence of serious animal-induced allergies among current zoo employees is lower than that among the general population receives the strongest support from the information given.