Thinking and Deciding

OTHER BIASES IN RISK JUDGMENTS 515
dren. Subjects were asked to indicate the maximum overall death rate for vaccinated
children for which they would be willing to vaccinate their child. Most subjects answered
well below 9 per 10,000. Of the subjects who showed this kind of reluctance,
the mean tolerable risk was about 5 out of 10,000, half the risk of the illness itself.
The results were also found when subjects were asked to take the position of a policy
maker deciding for large numbers of children. When subjects were asked for justifications,
some said that they would be responsible for any deaths caused by the
vaccine, but they would not be (as) responsible for deaths caused by failure to vaccinate.
In other studies, mothers who resist DPT vaccination for their children show
a lower mean tolerable risk than mothers who accept the vaccination (Asch, Baron,
Hershey, Kunreuther, Meszaros, Ritov, and Spranca, 1994; Meszaros et al., 1992).
Intuitive toxicology and naive theories
When people think about risks from the ingestion of chemicals, such as pesticides on
fruit, they have a set of intuitions similar to the zero-risk idea. Kraus, Malmfors, and
Slovic (1992) introduced the term “intuitive toxicology” for these beliefs. Toxicology
is the science of poisons and harmful chemicals. A kernel of wisdom from that
scientific field is that, in general, “the dose makes the poison.” In other words, most
chemicals are not harmful except in high doses. Moreover, most of the chemicals in
food, including vitamins and minerals, are harmful if the dose is high enough. And,
in general, the larger the dose, the greater the harm. The question of whether even
the most dangerous chemicals require some threshold before they do any harm is
hotly debated and has rarely been settled for any chemicals.
In contrast to this scientific wisdom, people have what amounts to naive theories
about harmful chemicals. In a questionnaire study of toxicologists and laypeople,
35% of the public agreed that “For pesticides, it’s not how much of the chemical you
are exposed to that should worry you, but whether or not you are exposed to it at all.”
Only 4% of the scientists agreed. Other questions supported the general conclusion
that lay people thought that chemical effects were dependent on exposure and were
not dependent on the dose (Kraus et al., 1992).
Lay people have similar theories about nutrition and harmful foods. In a survey
of students and other groups in the U.S., 20% agreed that, “If something can cause
harm to the body in large amounts, then it is always better not to eat it even in small
amounts” (Rozin, Ashmore, and Markwith, 1996). Likewise, 26% agreed that a diet
free of salt is more healthful than the same diet with a pinch of salt. Rozin and his
colleagues have argued that these results are related to a naive theory based on the
idea of contagion from contact. The same kind of thinking makes people reluctant
to wear a sweater that has been worn by someone who had AIDS, even though they
agree that AIDS cannot be transmitted through clothing, and even reluctant to wear
something that has been worn by Adolph Hitler. They refer to the laws of “sympathetic
magic,” which are apparently part of beliefs of many cultures throughout the
world. The main idea is, “Once in contact, always in contact.” Contagion is thus
seen as insensitive to dose.