JUDGING THE IMPORTANCE OF CONSTANT AND VARIABLE CANDIDATE CAUSES - A TEST OF THE POWER PC THEORY

In two causal induction experiments subjects rated the importance of p airs of candidate causes in the production of a target effect; one can didate was present on every trial (constant cause), whereas the other was present on only some trials (variable cause). The design of both e xperiments consisted of a factorial combination of two values of the v ariable cause's covariation with the effect and three levels of the ba se rate of the effect. Judgements of the constant cause were inversely proportional to the level of covariation of the variable cause but we re proportional to the base rate of the effect. The judgements were co nsistent with the predictions derived from the Rescorla-Wagner (1972) model of associative learning and with the predictions of the causal p ower theory of the probabilistic contrast model (Cheng, 1997) or ''pow er PC theory''. However, judgements of the importance of the variable candidate cause were proportional to the base rate of the effect, a ph enomenon that is in some cases anticipated by the power PC theory. An alternative associative model, Pearce's (1987) similarity-based genera lization model, predicts the influence of the base rate of the effect on the estimates of both the constant and the variable cause.