Preference for mixed versus constant delays of reinforcement: Effect of probability of the short, mixed delay.
Rats suddenly prefer mixed delays once a large share are short, a jump today's models still miss.
01Research in Context
What this study did
The team placed rats in a two-key chamber. One key led to a fixed delay before food. The other key led to a mix of short (.2 s) and long (8 s) delays.
They varied how often the short delay showed up in the mix. The rats could press either key at any time.
What they found
Once the short delay made up a large share of the mixed schedule, rats strongly preferred the mixed side. Below that, they picked the fixed side or were indifferent.
Standard math models (delay-reduction and harmonic-mean) predicted flat choice. The rats' steep jump in preference did not match those curves.
How this fits with other research
Schmitt (1984) ran a near-copy study the next year. They added more reinforcers and longer terminal links. Their rats also shifted choice with delay, showing the effect is reliable.
Delano (2007) tested pigeons in a successive-encounter task. Mixed versus fixed delays again changed choice, but the birds' data fit a hyperbolic decay model. The species difference may explain why the 1983 rat data did not fit the same math.
Guest et al. (2013) looked at brief unsignaled delays in VR versus VI schedules. They found higher response rates only in the VI part. Together these studies show that both schedule type and signal matter when you add any delay.
Why it matters
If you mix reinforcement times in a token board or DRO schedule, keep at least a large share of the deliveries short. That ratio can flip client preference toward the mixed option and maintain engagement. Also, treat current delay models as rough guides, not gospel—watch the data first.
Want CEUs on This Topic?
The ABA Clubhouse has 60+ free CEUs — live every Wednesday. Ethics, supervision & clinical topics.
Join Free →Program every fourth token or reinforcer to deliver immediately to hit the a large share short-delay mark and boost client preference.
02At a glance
03Original abstract
Preference for mixed versus constant delays of reinforcement was studied with a concurrent-chain procedure. Lever pressing by rats in concurrently available variable-interval 60-second initial links occasionally produced mutually exclusive terminal-link reinforcement delays. A constant delay of reinforcement (either 15 seconds or 30 seconds) composed one terminal link and mixed delays (.2 second and twice the value of the constant delay) were arranged in the other terminal link. The proportion of .2-second delays in the mixed-delay terminal link took on values of 0, .1, .25, .5, .75, .9, and 1.0 over experimental conditions. Based on relative rates of responding in the initial links, preference for the mixed delays was a negatively accelerated function of the proportion of short, mixed delays. Three of five rats preferred the mixed delays to the constant delays when the proportion of short, mixed delays was .1 or higher, and all five rats preferred the mixed delays when the proportion of short, mixed delays was .25 or higher. Neither Squires and Fantino's (1971) delay-reduction model of choice nor a model based on the harmonic mean reinforcement delay provided a close estimate of choice proportions over the range of short-delay proportions studied. The delay-reduction model underestimated choice for the mixed delays at low and intermediate proportions of short delays, and the harmonic-mean-delay model overestimated choice for the mixed delays at intermediate and high proportions of short delays.
Journal of the experimental analysis of behavior, 1983 · doi:10.1901/jeab.1983.39-257