Relationship between response rate and reinforcement frequency in variable-interval schedules: II. Effect of the volume of sucrose reinforcement.
Shrinking reinforcer size slides the VI response curve rightward—expect slower rates unless you raise payment frequency.
01Research in Context
What this study did
The team worked with six lab rats. Each rat pressed a lever for sugar water on a variable-interval schedule. The schedule paid the rat only for the first press after an unpredictable time gap.
They tested three sugar volumes: 0.05, 0.10, and 0.20 ml per drop. Every volume stayed in place for at least five sessions so steady performance could settle.
What they found
Smaller drops slid the whole response curve to the right. Rats needed about twice as many payments per hour to reach half of their top speed when the drop shrank from 0.20 ml to 0.05 ml.
Top speed itself did not move. Once payments were frequent enough, rats pressed just as fast for tiny drops as for big ones.
How this fits with other research
Rose et al. (2000) later showed the same pattern with sugar strength instead of size. Both studies confirm that weaker reinforcers only shift the curve sideways; they do not lower the ceiling.
Wilkie et al. (1981) part III swapped volume for d-amphetamine. Drugged rats also slid right, proving the sideways shift can come from many kinds of weakened reinforcement.
Davis et al. (1972) looks opposite at first: extra free payments slowed baseline pressing. The key difference is source. Extra payments from another lever act like competition, while smaller drops simply cheapen the payoff. Same equation, different knob.
Why it matters
If a client’s motivation feels low, first check the size of the payoff, not the task. A thinner slice of praise or shorter iPad turn may need twice as many deliveries to keep responding brisk. You can stay on a lean VI schedule, just pack more payouts into the hour or sweeten each one. Either fix keeps the ceiling high without extra response effort.
Want CEUs on This Topic?
The ABA Clubhouse has 60+ free CEUs — live every Wednesday. Ethics, supervision & clinical topics.
Join Free →Count current praise or token size; if you cut it, double the rate of delivery for the next two sessions and track response speed.
02At a glance
03Original abstract
Three rats were exposed to variable-interval schedules specifying a range of different reinforcement frequencies, using three different volumes of .32 molar sucrose (.10, .05, and .02 milliliters) as the reinforcer. With each of the three volumes, the rates of responding of all three rats were increasing, negatively accelerated functions of reinforcement frequency, the data conforming closely to Herrnstein's equation. In each rat the value of the constant K(H), which expresses the reinforcement frequency needed to obtain the half-maximal response rate, increased with decreasing reinforcer volume, the values obtained with .02 milliliters being significantly greater than the values obtained with .10 milliliters. The values of the constant R(max), which expresses the theoretical maximum response rate, were not systematically related to reinforcer volume. The effect of reinforcer volume upon the relationship between response rate and reinforcement frequency is thus different from the effect of the concentration of sucrose reinforcement: In a previous experiment (Bradshaw, Szabadi, & Bevan, 1978) it was found that sucrose concentration influenced the values of both constants, R(max) increasing and K(H) decreasing with increasing sucrose concentration.
Journal of the experimental analysis of behavior, 1981 · doi:10.1901/jeab.1981.35-263