Resistance to change produced by access to fixed-delay versus variable-delay terminal links.

Lord et al. (1986) worked with pigeons in a two-part chain.

Pecks in the first link opened a second link that always ended with food.

The second link gave either a fixed wait or a variable wait before the pellet dropped.

Birds experienced both types in mixed order so the team could compare strength of responding.

Variable delays kept the birds pecking faster than fixed delays of the same average length.

When the researchers later gave free food or stopped payoff, the variable-delay key also held out longer against the change.

In plain words, unpredictability glued the behavior to the key.

Leslie (1981) had already shown that local reinforcement probability steers moment-to-moment pace; the 1986 paper adds that the same factor governs long-run staying power.

Lecavalier et al. (2006) looked at the flip side: they found that even short delays before payoff can accidentally increase variability when you want repetition.

Together the two studies warn you—delays shape both persistence and form, so choose the schedule to match your teaching goal.

Older work like KELLEHEBERRYMAELLIOTT et al. (1962) proved that stimuli chained to FI schedules control timing; C et al. extend that idea by showing delay type, not just interval length, decides how hard the behavior fights change.

If you want a skill to survive satiation, breaks, or extinction, build in variable wait times before reinforcement.

Use a mix of quick and slower praise or tokens instead of a fixed five-second delay.

Watch for the flip side—too much variability in delay can also loosen the exact form you want, so check response shape as you tinker with timing.