Thinking and Deciding

OTHER BIASES IN RISK JUDGMENTS 513
cause cancer in test animals (Ames and Gold, 1990). Caffeic acid is a “carcinogen”
found in coffee, lettuce, apples, pears, plums, celery, carrots, and potatoes. In one
study, 94% of synthetic carcinogens were subject to U.S. government regulation, but
only 41% of a sample of natural carcinogens (Viscusi, 1995). Moreover, the average
risk to humans of the synthetic chemicals was lower than that of the natural ones.
This all assumes that the animal tests themselves are useful in detecting potential
carcinogens. Ames and Gold (1990) argue that the animal tests require such high
doses of the chemicals that the cancer is often caused by increased cell division,
which happens because the chemical starts poisoning cells and the remaining ones
start to divide more quickly. At lower doses of the same chemicals, there is no
cell poisoning. So the animal tests may be highly inaccurate. In sum, Ames and
Gold argue, if you break up carrots into their constituent chemicals and inject the
chemicals into rats in high doses, the rats may get cancer, but carrots are still good
for you, and all the evidence suggests that people who eat lots of fruit and vegetables
are reducing their risk of cancer. And, if the test is no good for natural chemicals, it
is probably not good for synthetic chemicals either.
The same consequences are evaluated differently depending on whether or not
people bring them about:
I don’t mind natural extinctions, but I’m not too enthusiastic about extinctions
that are directly caused by man. I feel that a species has a
right to survive and be able to survive on its own and be able to change
and evolve without the influence of whatever man does. I don’t want
to see man kill [any species]. If it’s going to happen, it should happen
naturally, not through anything that man has an influence on. (From a
respondent quoted by Kempton, Boster, and Hartley (1995), p. 109.)
Questionnaire studies show the same effects. People are more willing to pay
to reduce risks when the source of harm is human than when the source is natural
(Kahneman et al., 1993; Kahneman and Ritov, 1994). For example, subjects
were willing to contribute about $19 to an international fund to save Mediterranean
dolphins when the dolphins were “threatened by pollution” but only $6 when the
dolphins were “threatened by a new virus.” Similarly, subjects (including judges)
think that compensation for injuries such as infertility should be greater when the
injury is caused by a drug rather a natural disease, even if the penalty paid by the
drug maker does not affect the amount of compensation paid to the victim (Baron
and Ritov, 1993).
These questionnaire studies are important because they suggest that the observed
economic effects depend on psychological properties of human judgment, rather than
some unknown economic effects. In particular, people may use a heuristic principle,
or naive theory, that “natural is good.” This is a reasonable heuristic. Natural selection
has endowed biological systems in their natural environments with some stability,
that could be disrupted by changes. But this is a crude principle at best. Strict
adherence to it would have prevented the development of modern medicine, among