Humanistic psychology is a school of psychology that emerged in the 1950s in reaction to both behaviorism and psychoanalysis. The humanistic approach has its roots in existentialist thought (see Kierkegaard, Nietzsche, Heidegger, and Sartre). It is also sometimes understood within the concept of the three different forces of psychology; behaviorism, psychoanalysis and humanism. Behaviorism grew out of Ivan Pavlov's work with the conditioned reflex, and laid the foundations for academic psychology in the United States associated with the names of John B. Watson and B.F. Skinner. This school was later called the science of behavior. Abraham Maslow later gave behaviorism the name "the first force". The "second force" came out of Freud's research of psychoanalysis, and the psychologies of Alfred Adler, Erik Erikson, Carl Jung, Erich Fromm, Karen Horney, Otto Rank, Melanie Klein, Harry Stack Sullivan, and others. These theorists focused on the depth of the human psyche, which they stressed, must be combined with those of the conscious mind in order to produce a healthy human personality.