The effectiveness of inhibitors in human predictive judgments depends on the strength of the positive predictor.

We tested whether the development of inhibitory strength, as measured by a summation test, is proportional to the strength of the positive cue (P) against which the inhibitory cue (I) is trained. P predicted the outcome, whereas the co-occurrence of P with I (PI) predicted no outcome. In Experiments 1, 2, and 3, we compared the latter design against a version in which P was overshadowed by another cue (X). In this design, the compound P degrees X predicted the outcome, but P degrees I degrees predicted no outcome. In all three experiments, overshadowed cue I degrees was less inhibitory than I. In Experiment 4, a P produced by fewer training trials also supported weaker inhibitory learning. Overall, the results were consistent with associative learning theories, especially Pearce's (1994) configural model. Contingency models need to make additional assumptions to accommodate this property of inhibitory learning.