How to study cognitive decision algorithms: The case of the priority heuristic

Although the priority heuristic (PH) is conceived as a cognitive-process model, some of its critical process assumptions remain to be tested. The PH makes very strong ordinal and quantitative assumptions about the strictly sequential, non-compensatory use of three cues in choices between lotteries: (1) the difference between worst outcomes, (2) the difference in worst-case probabilities, and (3) the best outcome that can be obtained. These aspects were manipulated orthogonally in the present experiment. No support was found for the PH. Although the main effect of the primary worst-outcome manipulation was significant, it came along with other effects that the PH excludes. A strong effect of the secondary manipulation of worst-outcome probabilities was not confined to small differences in worst-outcomes; it was actually stronger for large worst-outcome differences. Overall winning probabilities that the PH ignores exerted a systematic influence. The overall rate of choices correctly predicted by the PH was close to chance, although high inter-judge agreement reflected systematic responding. These findings raise fundamental questions about the theoretical status of heuristics as fixed modules.