John Watson was an American Psychologist who defined Psychology as the Science of human behaviour.
He conducted research on animal behaviour, child-rearing, and advertising.
He explained human behaviour through the S-O-R Model.
Stimulus (S) refers to any physical event or condition that gives rise to a reaction. Organism (0) refers to a human being or an animal, who gives a response to the stimulus. Response (R) refers to the reaction of the organism to a given stimulus.
Watson also established the school of behaviourism.