WHEN predicting an event, people neglect overall frequencies (base rates) of various possibilities(1). We have previously shown(2) that this base-rate occurs not only with word problems, but also in a procedure with repeated trials : a sample cue followed by two choice options, one of which the subject must choose, with feedback regarding the correctness of the choice. Perhaps this base-rate error depends on people’s histories of matching physically similar items. In support of this suggestion, we begin by showing that the base-rate error is eliminated with physically unrelated items. We then show that when the relation between items is again arbitrary, but is a relation that subjects already know, the error reappears. Finally we show that teaching subjects new arbitrary relations reintroduces the error in later testing. These experiments demonstrate a fundamental base-rate error dependent on learned relationships : without interference from pre-existing associations between cues and options, predictions may be made more optimally.