Highly influential "dual-process" accounts of human cognition postulate the coexistence of a slow accurate system with a fast error-prone system. But why would there be just two systems rather than, say, one or 93? Here, we argue that a dual-process architecture might reflect a rational tradeoff between the cognitive flexibility afforded by multiple systems and the time and effort required to choose between them. We investigate what the optimal set and number of cognitive systems would be depending on the structure of the environment. We find that the optimal number of systems depends on the variability of the environment and the difficulty of deciding when which system should be used. Furthermore, we find that there is a plausible range of conditions under which it is optimal to be equipped with a fast system that performs no deliberation ("System 1") and a slow system that achieves a higher expected accuracy through deliberation ("System 2"). Our findings thereby suggest a rational reinterpretation of dual-process theories.
Keywords: Bounded optimality; Bounded rationality; Dual-process theories; Meta-decision making; Metareasoning; Resource-rationality.
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