The present experiment places subjects in an abstract medical diagnosis context, in which a similarity judgment is posed with the manipulation of the additive common symptom. Results obtained from eighty-nine subjects (31 males and 58 females) reveal that the (perplexing) inconsistent responses across the trials without, and with, the common symptom added, are not simply dependent on the symptom evidence, but rather jointly on the type of the common symptom and the pattern of responses to the trial without a common symptom. This result is consistent with the equate-to-differentiate explanation, but not with the independence account.

Key words: independence, equate-to-differentiate, medical decision making