Hypothesis Generation

Hypothesis Generation

Many judgments require people to generate hypotheses as well as assess their likelihood. In this account, the generation of alternative hypotheses entails some unpacking of the residual hypothesis and, thus, is expected to increase its support relative to the focal hypothesis. In the absence of explicit instructions to generate alternative hypotheses, people are less likely to unpack the residual hypothesis and thus tend to overestimate specified hypotheses relative to those left unspecified.

This implication has been confirmed by Gettys and his colleagues (Gettys, Mehle, & Fisher, 1986; Mehle et al., 1981), who have found that, in comparison with veridical values, people generally tend to overestimate the probability of specified hypotheses presented to them for evaluation. Indeed, overconfidence that one’s judgment is correct (e.g., Lichtenstein, Fischhoff, & Phillips, 1982) may sometimes arise because the focal hypothesis is specified, whereas its alternatives often are not. Mehle et al. (1981) used two manipulations to encourage unpacking of the residual hypothesis: One group of subjects was provided with exemplar members of the residual, and another was asked to generate its own examples. Both manipulations improved performance by decreasing the probability assigned to specified alternatives and increasing that assigned to the residual. These results suggest that the effects of hypothesis generation are due to the additional hypotheses it brings to mind, because simply providing hypotheses to the subject has the same effect. Using a similar manipulation, Dube-Rioux and Russo (1988) found that generation of alternative hypotheses increased the judged probability of the residual relative to that of specified categories and attenuated the effect of omitting a category. Examination of the number of instances generated by the subjects showed that, when enough instances were produced, the effect of category omission was eliminated altogether.

Now consider a task in which subjects are asked to generate a hypothesis (e.g., to guess which film will win the best picture Oscar at the next Academy Awards ceremony) before assessing its probability. Asking subjects to generate the most likely hypothesis might actually lead them to consider several candidates in the process of settling on the one they prefer. This process amounts to a partial unpacking of the residual hypothesis, which should decrease the judged probability of the focal hypothesis. Consistent with this prediction, one study (Koehler, 1994) found that subjects asked to generate their own hypotheses assigned them a lower probability of being true than did other subjects presented with the same hypotheses for evaluation. The interpretation of these results – that hypothesis generation makes alternative hypotheses more salient – was tested by two further manipulations. First, providing a closed set of specified alternatives eliminated the difference between the generation and evaluation conditions. In these circumstances, the residual should be represented in the same way in both conditions. Second, inserting a distracter task between hypothesis generation and probability assessment was sufficient to reduce the salience of alternatives brought to mind by the generation task, increasing the judged probability of the focal hypothesis.