Every observable action an individual demonstrates results in some type of outcome. When this outcome is highly pleasurable, there is a tendency to repeat the action in order to obtain the payoff again. When it is understood that the payoff will reliably occur, the action tends to be repeated even more often. These events basically describe what happens when behavior is positively reinforced.
Positive reinforcement is best when it occurs naturally from the environment. This is called natural reinforcement. For most people, this is how much of learning occurs – as a result of natural consequences within the context of day to day life. However, when learning is new or difficult, an extra incentive may sometimes be necessary in order to increase or sustain the effort. When an incentive is intentionally added in to increase performance, it is called contrived reinforcement.
The use of contrived reinforcement is almost always necessary with learners on the autism spectrum. Because people with ASD experience the world in very different ways than those who do not have the disorder, the kinds of things they find reinforcing are often atypical and frequently unavailable from the natural environment. Further, since learners with autism generally have problems figuring out what is and is not relevant in the environment, they often do not notice that naturally reinforcing opportunities even exist. With a tendency to draw illogical conclusions from the environment, the use of contrived reinforcement for learners on the spectrum can provide clarity during new or difficult learning with regard to what they are being reinforced for.
Excerpt from Autism and ABA: A "How-To" Handbook for Teachers