Clarifying the role of an unavailable distractor in human multiattribute choice

Abstract

Decisions between two alternatives can be influenced by a third irrelevant decoy alternative. Although decoy effects typically depend on the decoy's relative position in a multiattribute choice space, recent studies using risky prospects (i.e., varying in reward and probability) reported a novel 'positive' decoy effect operating on a single value dimension: The higher the expected value of an unavailable (distractor) prospect was, the easier the discrimination between two available target prospects became, especially when their expected value difference was small. Here we ask whether this previously reported distractor effect affords alternative interpretations. Indeed, we find that it occurred because the distractor's expected value covaried positively with the subjective utility difference between the two targets. Looking beyond this covariation, we report a modest negative distractor effect, which in turn could conceivably be attributed to classic multiattribute decoy effects. Our findings clarify the modulatory role of an unavailable distracting option in multiattribute decisions.