Behaviorism is a theoretical orientation based on the premise that scientific psychology should study only observable behaviors.   Behavior refers to any overt (observable) response or activity by an organism.  It flourished in the 1950s and 60s.  Behaviorism brought out the old age debate about Nature versus Nurture.  Nature being genetic inheritance and Nurture being our environment and experiences.  Behaviorists argued that our nurture was more important.  The used the stimulus-response psychology approach.  It attempts to relate overt behaviors (response) to observable events in the environment (stimuli).  The emergence of behaviorism was partly attributable to Ivan Pavlov, a Russian psychologist.   

John B Watson (1878-1958) founded the school of behaviorism.  He proposed that psychologists should abandon the study of the consciousness altogether.  Watson wanted psychology to be a science with reliable, exact knowledge, which would replace vague speculation and personal opinion.  He argued in the Nature vs. Nurture debate that people were made, not born.

Gestalt Psychology, in Germany, challenged Behaviorism's ideas.  They were concerned with perception.  People in this field wanted to continue to study the conscious experience rather than overt behavior.  Sigmund Freud, an Austrian psychologist, was also challenging the ideas of behaviorists, with the unconscious mental process. 

B. F. Skinner (1904-1990) campaigned a return to a strict focus on observable behavior.  He didn't deny internal mental events but insisted that they couldn't be studied scientifically.  He put much emphasis on how environmental factors mold our behavior.  Skinner documented the fundamental principle of behavior: Organisms tend to repeat responses that lead to positive outcomes, and they tend not to repeat responses that lead to neutral or negative outcomes.  Skinner worked with rats and pigeons to show that he could exert control over behavior of animals by manipulating the outcomes of their responses.  In 1971, he wrote a book, Beyond Freedom and Dignity.  Skinner also said that people are controlled by their environments not themselves, free will is an illusion.

Behavioral principles are now widely used in factories, schools, prisons, mental hospitals, and a variety of other settings.