The behaviour of humans and non-human animals can be understood in fairly simple terms if they act as rational decision makers. It has long been known, however, that humans do not always display economically rational behaviour, and recently it has been demonstrated experimentally that foraging birds and insects sometimes show irrational instability of preferences when choosing among food items. Theories of bounded rationality attempt to explain human departures from strictly rational behaviour by positing constraints on information gathering and processing, but these ideas do not question the fundamental (if practically unobtainable) optimizing nature of rationality. Biology offers an alternative hypothesis:  rational decision making has not evolved because rationality does not maximize fitness. We have addressed this idea with a simple simulation model in which agents have perfect information about the (deterministic) value of alternative choices in an arena with two biologically realistic features: stochastic fluctuation in the alternatives available at any given time, and assessment of final (lifetime) fitness after repeated rounds of decision making. Some agents are given stable preference structures and make individually rational choices given those structures; other agents construct their preference structures based on the suite of choices they face, in accord with a proposed description of human decision making known as Prospect Theory. We varied the number of alternative choices presented simultaneously and the number of rounds of choice before fitness assessment in the simulations.

Results/Conclusions

The results show that the potentially non-rational Prospect Theory decision maker has higher mean fitness than a rational decision maker for some circumstances in this ecologically simple setting. Adding further biological realism, such as a metabolic rate that must be fuelled by the resources selected in each round, and random severe “droughts” that deplete stored resources, tends to enhance the fitness advantage of the Prospect Theory agent. These results suggest that an only slightly “natural” context to decision making can render rationality less fit than some non-rational alternatives. Understanding how humans arrive at decisions will be essential if we hope to ameliorate the effect of millions of individual consumer decisions on the global environment.