Schedule-induced polydipsia as a function of fixed interval length.

Falk (1966) watched rats drink water while food arrived on fixed-interval schedules.

The team lengthened the FI from seconds up to 300 s and recorded every lick.

They wanted to see if longer waits between pellets changed how much water the rats drank.

Water intake rose in a straight line as the FI grew.

At the longest 300-s interval the rats drank the most, showing clear schedule-induced polydipsia.

Nelson et al. (1978) repeated the setup and pinned down timing: most drinks happened right after the pellet landed.

Blackman (1970) pushed the idea further by showing the same rise under FT and FR schedules, proving interval length—not accidental reinforcement—drives the effect.

Bacon-Prue et al. (1980) swapped dry pellets for salty liquid food and still saw polydipsia peak near 120 s, showing the rule holds across reinforcer types.

O'Leary et al. (1979) moved the paradigm to humans and found adjunctive locomotion instead of drinking, proving the schedule, not the species, creates the extra behavior.

When you stretch the time between reinforcers, clients may generate adjunctive behaviors—pacing, doodling, or even excessive drinking.

Spot these side effects early and plan alternative responses or shorter intervals to keep sessions on track.