He was associated with the Menninger Foundation, based in Kansas, between 1946-52 under David Rapaport's tutelage in psychological testing and Freudian theory.
[3][4] In 1953, Klein joined the New York University Department of Psychology's newly founded Research Center for Mental Health (RCMH) as co-director with Robert R.
Under their direction psychological researchers experimented with "subliminal stimuli", "cognitive control", assessment of primary process thinking, and altered states of consciousness (i.e. "sensory deprivation", "LSD", "REM sleep").
[6][7][8] Over the years, Klein held visiting professorships at several major universities, including Harvard, Berkeley, Chicago, Brandeis, and Clark.
In the last decade of his life, Klein was deeply immersed in an attempt to disentangle the duality of explanations inherent in psychoanalytic theory, the experience-near clinical level from its abstract base, its metapsychology Designating the clinical level a "theory of personal encounter", Klein proposed the cognitive concepts—such as "meaning", "awareness", "peremptory ideation, and intentionality" as the more appropriate explanatory terms than the natural-science based concepts of drives, energies and mechanisms.