Clarifying the role of an unavailable distractor in human multiattribute choice

Abstract

Decisions between two economic goods can be swayed by a third unavailable decoy alternative, which does not compete for choice, notoriously violating the principles of rational choice theory. Although decoy effects typically depend on the decoys 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 show that this unidimensional distractor effect affords alternative interpretations: it occurred because the distractors expected value covaried positively with the subjective utility difference between the two targets. Looking beyond this covariation, we report a modest negative distractor effect operating on subjective utility, as well as classic multiattribute decoy effects. A normatively meaningful model (selective integration), in which subjective utilities are shaped by intra-attribute information distortion, reproduces the multiattribute decoy effects, and as an epiphenomenon, the negative unidimensional distractor effect. These findings clarify the modulatory role of an unavailable distracting option, shedding fresh light on the mechanisms that govern multiattribute decisions.