Testing the evolutionary theory of behavior dynamics' predictions about choice under concurrent random‐ratio schedules
People pick the richer RR button more often when the ratio difference is large and the richer ratio is small—close to ETBD math, yet each person spreads around that line.
01Research in Context
What this study did
Blakemore et al. (2025) asked adults without disabilities to press two buttons. Each button paid points on a random-ratio (RR) schedule. The ratio on the left and right could differ.
The team wanted to know if people’s choices matched the numbers spit out by the Evolutionary Theory of Behavior Dynamics (ETBD). They kept the ratio difference small in some blocks and large in others.
What they found
When the ratio gap grew, people leaned harder on the richer side. When the richer side asked for fewer presses, the lean got even stronger.
Most data moved in the same direction the ETBD predicted, but one person might show a big swing while the next stayed near 50-50.
How this fits with other research
Renne et al. (1976) showed humans can match on VI schedules. Blakemore moves the test to RR schedules and still sees matching, so the law stretches across schedule types.
Szempruch et al. (1993) found only 13 of 30 adults matched on VI; the rest under-matched or went exclusive. Blakemore’s RR data echo that noise—people again scatter around the model line.
Winett et al. (1991) saw choice shift from matching to maximizing after brief training. Blakemore did not train, yet the average still slides toward maximizing as ratio differences grow, hinting the shift can emerge without extra teaching.
Why it matters
If you run concurrent schedules in your lab or classroom, expect people to favor the easier side more as the work gap widens. The ETBD gives you a quick formula to set ratios and forecast where most learners will land, but always plot individual data—some will buck the trend. Try thinning the richer schedule slowly; the model says preference should hold as long as the ratio gap stays wide.
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02At a glance
03Original abstract
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.
Journal of the Experimental Analysis of Behavior, 2025 · doi:10.1002/jeab.70065