Instrumental conditioning is a learning process in which voluntary behavior is modified through association with the addition (or removal) of rewarding or aversive stimuli. The frequency or duration of behavior may increase with reinforcement or decrease with punishment or extinction. Operant conditioning was developed by Edward Thorndike, whose law of effect suggested that behavior arises as a result of consequences, whether satisfaction or discomfort. In the 20th century, operant conditioning was studied by behavioral psychologists, who believed that many aspects of the psyche and behavior are explained by environmental conditioning. Reinforcement is environmental stimuli that reinforce behavior, while punishment is stimuli that weaken behavior. Both types of stimuli can be divided into positive and negative, which respectively involve the addition or removal of environmental stimuli. Operant conditioning differs from classical conditioning both in mechanism and in result. Whereas classical conditioning combines stimuli to form involuntary, reflex behavior (e.g., drooling when eating), operant conditioning shapes voluntary behavior through its consequences. Actions followed by rewards tend to be repeated, while actions followed by negative impacts are reduced. In the 20th century, the study of animal learning mainly focused on the analysis of these two types of learning, and they still form the basis of behavioral analysis. They have also been applied to the study of social psychology, helping to clarify some phenomena, such as the false consensus effect.