Testing the evolutionary theory of behavior dynamics' predictions about choice under concurrent random‐ratio schedules

The evolutionary theory of behavior dynamics (ETBD) has predicted that under concurrent random-ratio (RR) schedules, preference for the denser schedule becomes more extreme with (a) larger differences between the concurrent ratio requirements and (b) smaller absolute values of the ratio requirement for the denser alternative. In this study, we tested ETBD's predictions by evaluating human participants' choice under various concurrent RR schedules. Sixty-three undergraduate students participated and were presented with two concurrently available response options on a touchscreen monitor. The difference between the concurrently available ratio requirements was manipulated across conditions, and the absolute value of the ratio requirement for the denser alternative was manipulated across groups. As predicted by the ETBD, participants' preference for the denser alternative increased as the difference between the concurrent ratio requirements increased and groups with smaller absolute ratio requirements tended to display more extreme preference. However, a high level of heterogeneity was observed across human participants within each group that was not evident in the behavior of artificial organisms animated by the ETBD. Our findings demonstrate the importance of focusing on individual behavior and suggest directions for future research investigating choice under concurrent ratio schedules and evaluating the ETBD.