Evidence for deductive reasoning in blocking of causal judgments

We have recently demonstrated that pre-training of additivity (the outcome of two causal cues is larger than one causal cue) greatly enhances blocking. This manipulation could work by removing a ceiling effect on the outcome, as proposed by Cheng (1997). Alternatively, it could remove the logical ambiguity associated with blocking under non-additive conditions, thus permitting blocking as a deductive inference. We used a counterintuitive combination rule-subtractivity rather than additivity-to discriminate between these two accounts. Prior to a backward blocking causal judgment procedure (AB+, A+), we trained participants that a compound of one causal and one non-causal cue leads to the outcome, but that a compound of two causal cues leads to no outcome. This design allows the logical deduction that cue B is non-causal (blocking), but retains the ceiling effect on outcome magnitude. Blocking was strong following both subtractivity and additivity training, supporting the deductive reasoning account. Crown Copyright (C) 2004 Published by Elsevier Inc. All rights reserved.