Psychological aspects of genetic counseling. IV. The subjective assessment of probability.

Subjects asked to compare mathematically equal statements (or frames) of probability invariably perceived a difference in magnitude between percentages and equivalent proportions. Presented as unrelated to genetic risk, percentages tended to be chosen as having greater magnitude than their equivalent proportions. In justifying their choices subjects used a process of reasoning in which numbers were mentally compared, contrasted, or otherwise manipulated (numerical strategy). When framed as genetic risks, two types of reasoning were used to assess probabilities, the numerical strategy and one involving mental images of persons or parts of persons (person strategy). Subjects who used numerical reasoning tended to perceive percentages as having a greater risk magnitude than equivalent proportions whereas when person reasoning was used proportions with denominators of 10 or less tended to be perceived as having a greater risk magnitude than equivalent percentages. The implications of the findings for genetic counseling are discussed.