Risk should be objectively defined: comment on Pelé and Sueur

Pelé and Sueur (2013) propose that optimal decisions depend on delay of reinforcement, accuracy (probability or magnitude of reinforcement), and risk. The problem with this model is delay and accuracy are easy to define, but according to Pelé and Sueur the third, risk, depends on the animal's perceived or "interpreted" risk rather than actual (experienced) risk. Thus, choice of the smaller more immediate reward over the larger delayed reward (the delay discounting function) is viewed by the authors as optimal because delay is associated with increased risk (due to potential competition or predation). But perceived risk is assessed by the decision made (e.g., the slope of the discounting function), and since there is virtually no actual risk involved, by default if there is no independent means of measuring risk, according to Pelé and Sueur, all choices can be viewed as optimal. Thus, optimality is an untestable concept. We suggest that risk be defined by the actual risk (given sufficient experience to judge it) and under conditions in which there is no actual risk (or risk is controlled), when animals choose an alternative that provides a lower rate of access to food, that one considers such choice to be suboptimal.