Although strict behaviorists avoided all emotional terms, learning theory fully depends on emotion. In Thorndike’s Law of Effect and the theory of operant conditioning that followed, the likelihood that a behavior will be increased or decreased depends on its emotional impact. Behavior is increased when it elicits positive emotion (reward) and reduced when it elicits negative emotion (punishment). While it is more difficult to speak of emotions in animals, modern scientists assume that the simple emotional processes involved in learning theory—that is, forms of pleasure and pain—apply to both animals and humans.