Successive approximation

The core mechanic of shaping: rewarding behaviours that get progressively closer to the final goal.

Each step is reinforced until it is reliable, then the criterion is raised and only the next-closer approximation to the final behaviour is reinforced. Teaching a parrot to step into a carrier might involve reinforcing approach to the carrier, then a foot on the threshold, then a foot inside, then the whole body inside, then standing calmly inside with the door closed.

Successive approximation is the technical name for what most people mean when they talk about “shaping”. The two terms are largely interchangeable in practical use, though “shaping” emphasises the construction of the final behaviour while “successive approximation” emphasises the small-step mechanism that makes shaping work.

The technique is used across species. Zoo husbandry training relies heavily on successive approximation to build voluntary cooperation behaviours (presenting a paw for a blood draw, opening the mouth for an oral exam, holding still during ultrasound). Dog sports use successive approximation to build complex chained behaviours (agility courses, scent work, dance routines). Horse work uses successive approximation to build everything from float loading to advanced dressage movements.

Skilled application of successive approximation depends entirely on choosing the right size of step for the individual learner. Steps that are too small produce slow progress and can frustrate the animal; steps that are too large produce failure and lost reinforcement opportunities. The right step size is the largest one the animal can succeed at reliably, and it varies between individuals, between species, and between days for the same animal.

The technique is generally taught and applied using positive reinforcement, often with a bridging signal, but the principle works with any reinforcer.