Suboptimal choice in nonhuman animals: rats commit the sunk cost error.

The present experiments investigated the sunk cost error, an apparently irrational tendency to persist with an initial investment, in rats. This issue is of interest because some have argued that nonhuman animals do not commit this error. Two or three fixed-ratio (FR) response requirements were arranged on one lever, and an escape option was arranged on a second lever. The FRs were of different sizes, and escaping was the behavior of interest. Several variables that might influence the decision to persist versus escape were manipulated: the number of trials with different FR schedules in an experimental session (Exps. 1 and 2), effort to escape (Exp. 2), and the size of the larger FR (Exp. 3). The sunk cost error would result in never escaping, and the optimal strategy would be to escape from the larger FR. The main variable that determined persisting versus escaping was the size of the large FR. Rats that escaped from the large FR-apparently optimal behavior-did so at a suboptimal point, and hence committed the sunk cost error.