In this paper I examine Don Ross’s application of unificationism as a methodological criterion of theory appraisal in economics and cognitive science. Against Ross’s critique that explanations of the preference reversal phenomenon by the ‘heuristics and biases’ programme is ad hoc or ‘Ptolemaic’, I argue that the compatibility hypothesis, one of the explanations offerd by this programme, is theoretically and empirically well-motivated. A careful examination of this hypothesis suggests several strengths of a procedural approach to modelling cognitive processes underlying individual decision making, compared to a multiple-agent approach which Ross promotes. I argue that the debate between economists and psychologists are both theoretical and empirical, but cannot be resolved by appealing to the ideal of unification.