Pigeons keypecked on a two‐key procedure in which their choice ratios during one time period determined the reinforcement rates assigned to each key during the next period (Vaughan, 1981). During each of four phases, which differed in the reinforcement rates they provided for different choice ratios, the duration of these periods was four minutes, duplicating one condition from Vaughan's study. During the other four phases, these periods lasted six seconds. When these periods were long, the results were similar to Vaughan's and appeared compatible with melioration theory. But when these periods were short, the data were consistent with molecular maximizing (see Silberberg&Ziriax, 1982) and were incompatible with melioration, molar maximizing, and matching. In a simulation,statbirds following a molecular‐maximizing algorithm responded on the short‐ and long‐period conditions of this experiment. When the time periods lasted four minutes, the results were similar to Vaughan's and to the results of the four‐minute conditions of this study; when the time periods lasted six seconds, the choice data were similar to the data from real subjects for the six‐second conditions. Thus, a molecular‐maximizing response rule generated choice data comparable to those from the short‐ and long‐period conditions of this experiment. These data show that, among extant accounts, choice on the Vaughan procedure is most compatible with mol