This paper examines the development of British psychiatrist and psychoanalyst John Bowlby's views and their scientific and social reception in the United States during the 1950s. In a 1951 report for the World Health Organization Bowlby contended that the mother is the child's psychic organizer, as observational studies of children worldwide showed that absence of mother love had disastrous consequences for children's emotional health. By the end of the decade Bowlby had moved from observational studies of children in hospitals to animal research in order to support his thesis that mother love is a biological need. I examine the development of Bowlby's views and their scientific and social reception in the United States during the 1950s, a central period in the evolution of his views and in debates about the social implications of his work. I argue that Bowlby's view that mother love was a biological need for children influenced discussions about the desirability of mothers working outside the home during the early Cold War. By claiming that the future of a child's mind is determined by her mother's heart, Bowlby's argument exerted an unusually strong emotional demand on mothers and had powerful implications for the moral valuation of maternal care and love.