Expectancy-value models of health behaviour: An analysis by conjoint measurement

Various studies in the health area consistently rejected the multiplicative combination between severity and probability of threat which is predicted by expectancy-value (EV) theories. It is hypothesized here, that this negative evidence may be due to an overly demanding assumption underlying the multiplicative combination, namely, the assumption that people are able to performs trade-offs between expectancies and valences. This hypothesis is tested in two studies in which subjects judged hypothetical health threats. Results from a nonparametric analysis (conjoint measurement) of individual data (Study 1) and an experimental study of trade-off judgments (Study 2) are mostly consistent with the prediction. Unexpectedly, however, an ANOVA of the aggregate data of Study 1 yielded a small, but significant effect consistent with the multiplicative assumption. Whereas this latter result can be interpreted as evidencing an attempt to perform trade-offs, the overall results show as predicted that trade-off judgments are associated with a systematic error component due to the inherent difficulty of this type of judgment.