Reference: B.F. Skinner: all human behaviour is the product of operant conditioning.
Reference: B.F. Skinner: "When a bit of behaviour is followed by a certain kind of consequence, it is more likely to occur again, and a consequence having this effect is called a reinforcer."
Reference: B.F. Skinner: the process of operant conditioning as a consequence of evolution: "The process of operant conditioning presumably evolved when those organisms which were more sensitively affected by the consequences of their behaviour were better able to adjust to the environment and survive."
