How does repetition in varied environments develop a horse's reliability?

A horse that has learned a skill in a single, familiar environment has not fully generalized that skill — it has learned to perform the behavior in that specific context, and it may fail to produce the same response when the context changes. This phenomenon is well recognized in horse training and is one of the reasons horses that perform reliably at home often appear to forget their training at a show or in an unfamiliar arena. The skill has been learned but not generalized across environments. Generalizing a learned behavior requires repetition of that behavior in a variety of different settings, with varying levels of distraction and environmental novelty, until the horse produces the correct response reliably regardless of where it is or what is happening around it. This process is sometimes called proofing the behavior, and it is accomplished by deliberately introducing new practice environments after the skill is confirmed in the familiar one. A horse that has learned to stop correctly in its home arena should be asked to stop correctly in a different arena, in a field, near other horses, and in the presence of distractions before its stop can be considered genuinely reliable. Each new environment in which the horse produces the correct response adds to the breadth of its learning and increases the probability that the response will be available in any environment the horse encounters. Repetition in varied environments is the final stage of training a skill to true reliability, and it is a stage that is frequently skipped by trainers who move too quickly from basic acquisition of a behavior to competition demands.