Fixed-interval behavior: effects of percentage reinforcement.

Byrd (1972) asked a simple question: how little food do pigeons need to keep the fixed-interval scallop alive?

Birds pecked a key on an FI schedule while the experimenter dropped the percentage of intervals that ended with grain.

Some birds worked in a lit chamber, others in the dark, to see if vision changed the result.

The classic scallop survived even when only 7 % of intervals paid off.

In the light, response rates first rose then fell as reinforcement got thinner.

In the dark, rates simply dropped the less food they got, but the scallop shape still showed up whenever grain finally arrived.

Whitehead et al. (1975) removed the peck-to-eat link altogether and saw the scallop flatten. Byrd (1972) kept the link but paid only rarely; the scallop stayed. Together they map the boundary: contingency matters more than payoff density.

Rogers-Warren et al. (1976) added free food on the side and also flattened the pattern. D’s rare food still came after the interval, so the temporal cue stayed intact. Again, contingency plus timing beats sheer rate of reinforcement.

Bijou (1958) showed preschoolers keep an FI pattern even during extinction. D pushed pigeons to the same limit—schedule-induced patterns are hard to kill once they form.

If you run thin schedules in token boards or classroom timing systems, you can trust that the learner’s rhythm will hold even when payoffs are scarce. Keep the response-reinforcer link and the time cue; you can fade to 1 in 14 intervals without losing the pattern. That saves reinforcers while keeping behavior steady.