The conjunction fallacy

Abstract

The conjunction fallacy (CF) comes about when the occurrence of two events is rated as more likely than either in isolation. A typical participant in a CF study is presented with a description of a hypothetical individual (i.e., a compound sample stimulus) and then asked to make judgments as to the likelihood that that person engages in a particular vocation, avocation (i.e., single comparison stimuli), or both (i.e., a compound comparison stimulus). The CF is witnessed when the combination is judged as more likely than either the vocation or avocation alone. Commission of the CF is often attributed to participants’ judgment being guided by representativeness (i.e., the representativeness heuristic) rather than the laws of probability (Tversky & Kahneman, 1983). This paper provides a behavioral interpretation of CF responding based on derived/emergent stimulus relations and presents data from 27 undergraduates who participated in a study designed to test the plausibility of the interpretation. Using nonsense words and a many-to-one training structure, prerequisite baseline relationships for establishing two 6-member stimulus equivalence classes were trained. Next was an analogue CF test in which participants rated the likelihood that former sample stimuli (now presented as comparison stimuli, either alone or in sets of 2) were a correct answer given a compound sample stimulus composed of 3 former sample stimuli (all of which were from 1 class but had never been directly related during training). Following this test phase the emergence of equivalence classes was formally assessed and a standard CF scenario presented. The incidence of CF responding under the analogue procedure (60%) was similar to that reported in standard scenarios and commission of the CF was significantly related to whether equivalence classes formed. These data constitute a preliminary demonstration in support of the derived equivalence relations interpretation.