By the late 1950s, he had accumulated a body of observational and theoretical work to indicate the fundamental importance for human development of attachment from birth. Bowlby was interested in finding out the patterns of family interaction involved in both healthy and pathological development. He focused on how attachment difficulties were transmitted from one generation to the next. In his development of attachment theory, he proposed the idea that attachment behaviour was an evolutionary survival strategy for protecting the infant from predators. Mary Ainsworth joined Bowlby's research unit at Tavistock and further extended and tested his ideas. She played the primary role in suggesting that several attachment styles existed. Seven important experiences for Bowlby's future work and the development of attachment theory were: Bowlby's teaching experience at Priory Gates School, where he worked with maladapted and delinquent children. His opportunity to work with children who were evacuated from their families due to the war, leading to his work on the forty-four juvenile thieves. The addition of an ethology perspective to his thoughts and observations of mother-child separations, which helped him move beyond a psychoanalytic perspective. Mary Ainsworth's structured observation technique known as strange situation and the development of the different types of attachment styles as well as her contributions and introduction of the secure base to Bowlby. James Robertson (in 1952) in making the documentary film A Two-Year Old Goes to Hospital, which was one of the films about "young children in brief separation". The documentary illustrated the impact of loss and suffering experienced by young children separated from their primary caretakers. This film was instrumental in a campaign to alter hospital restrictions on visiting by parents. In 1952 when he and Robertson presented their film A Two Year Old Goes to Hospital to the British Psychoanalytical Society, psychoanalysts did not accept that a child would mourn or experience grief on separation but instead saw the child's distress as caused by elements of unconscious fantasies (in the film because the mother was pregnant). Bowlby also incorporated Robertson's naturalistic observation methods of children's behaviors. Melanie Klein during his psychoanalytic training. She was his supervisor; however, they had different views about the role of the mother in the treatment of a three-year-old boy. Specifically and importantly, Klein stressed the role of the child's fantasies about his mother, but Bowlby emphasised the actual history of the relationship. Bowlby's views—that children were responding to