How similarity between choice options affects decisions from experience: the accentuation-of-differences model

Traditional theories of decision making require that humans evaluate choice options independently of each other. The independence principle underlying this notion states that the relative choice probability of two options should be independent of the choice set. Previous research demonstrated systematic violations of this principle in decisions from description (context effects), leading to the development of various models explaining them. Yet, the cognitive processes underlying multi-alternative decisions from experience remain unclear. Furthermore, it is not known whether context effects also occur in such decisions, and existing learning models do not predict them. In three experiments, the similarity effect, compromise effect, and attraction effect were explored in a 3-armed bandit task with full feedback. Participants' behavior systematically violated the independence principle, although mostly not in line with past context-effect patterns in decisions from description. The observed similarity effect and the reversals of the compromise and the attraction effects can be explained by a similarity mechanism, according to which options with similar outcomes appear less attractive. We propose the accentuationof-differences model that relies on this mechanism. We further validated the model in a fourth experiment in which we demonstrated a new violation of independence, the accentuation effect. Across all experiments, the model outperformed traditional reinforcement-learning models in describing the observed findings. Finally, the model's generalizability was confirmed using the Iowa gambling task. In summary, the present work is the first to demonstrate systematic violations of the independence principle in various decisions-from-experience designs and to offer a model to explain the observed phenomena.