Avoidance of a return to the first component of a chain from the terminal component.

Nigro (1966) built a two-part chain. Pigeons pecked in the second part to postpone going back to the first part.

The team made the first part longer or shorter. They also changed how many pecks the birds had to give. Then they watched how hard the birds worked to stay out of that first part.

Sometimes a longer first part made the birds work more. Other times it made them work less. The same flip happened when the required pecks went up.

There was no straight line. Avoidance jumped around as the schedule numbers changed.

SIDMAN (1962) showed monkeys will pull a chain to earn a break from avoidance. Nigro (1966) moves the idea forward: the break does not have to be quiet time; simply staying in the later part is enough.

Mann et al. (1971) later asked if the break still works when no peck is needed to get it. They found it does, but only after the birds first learned that pecks once earned the break. Together the three papers draw a curve: timeout is good, earned timeout is better, and avoiding an earlier task can stand in for either.

Kendall (1967) used the same two-part FI chain. He saw response rates fall when the first FI grew. Nigro (1966) saw avoidance rates go both up and down. The difference is the task: B measured how fast pigeons peck for food; R measured how hard they work to skip a segment. Same schedule, different reinforcer, different story.

Your client may work to escape the earlier, harder part of a task chain. If you see big swings in effort after small schedule changes, think avoidance. Try making the early part shorter or sweeter before you add more prompts. The reinforcer is the chance to stay where they are.