In this contribution, the authors give an overview of the different studies on the effect of separation and deprivation that drew the attention of many in the 1940s and 1950s. Both Harlow and Bowlby were exposed to and influenced by these different studies on the so called 'hospitalization' effect. The work of Bakwin, Goldfarb, Spitz, and others is discussed and attention is drawn to films that were used to support new ideas on the effects of maternal deprivation.Keywords Separation . Maternal deprivation . Hospitalization effect . History of psychology . Attachment theory . Harlow . BowlbyFrom the 1930s through the 1950s clinical and experimental psychology were dominated by ideas from Freud's psychoanalytic theory and Watson's behaviorism. Although very different in their approach to the study of (human) behavior, psychoanalysts and behaviorists held common views on the nature of the bond between mother and infant. According to scientists from both disciplines the basis for this relationship was a secondary drive, i.e. the fact that the child valued and loved the mother was because she reduced his or her primary drive for food.The central figure of this special issue, American animal psychologist Harry Harlow , in the 1950s shifted his focus from studies of learning in monkeys (e.g., Harlow and Bromer 1938;Harlow 1949) to a more developmental approach-or in Harlow's own words a transition "from learning to love" (cf. Harlow and Harlow 1986). Inspired by the work of René Spitz (1945 Wolf 1946, 1949) on hospitalization, Harlow started his groundbreaking experiments on the effects of separation and deprivation in infant rhesus monkeys (e.g., Harlow 1958;Harlow and Zimmermann 1959). With his cloth and wire mother Integr Psych Behav (2008) 42:325-335