The motivational properties of schedule-induced polydipsia.

Falk (1966) watched rats work for food on a VI 1-min schedule. Every minute, on average, a food pellet dropped. Water was always there, yet the rats began to drink a lot—way more than they needed.

The team called this extra drinking “schedule-induced polydipsia.” They wanted to show it was the food schedule, not simple thirst, driving the behavior.

Polydipsia showed up fast. When the schedule ran, drinking soared. When water was harder to reach, the rats still drank, just less often. The pattern proved the drinking was adjunctive—an extra behavior glued to the food schedule.

CATANIDINSMOOR (1962) saw the same post-food drinking four years earlier, but called it “adventitious.” Falk (1966) gave the effect its now-famous name and clearer proof.

Morris et al. (1982) later asked why the drinking happens. They ruled out simple food cues and showed it emerges from competition among motivated behaviors, sharpening the explanation L started.

Wilson et al. (1987) swapped water for food mash and got schedule-induced overeating in already-full rats. The same schedules that make rats drink too much can make them eat too much—same mechanism, different topography.

If you run dense reinforcement schedules, watch for extra behaviors that tag along. A client might drink, eat, or even pace more right after reinforcers—not because they’re thirsty or hungry, but because the schedule invites it. Check for these adjunctive behaviors when you see sudden, unexplained spikes in water intake or snack requests. Thinning the schedule or adding response effort can keep the target skill strong while trimming the side behavior.