As a clinician, Bowlby had taken an interest in the impact of the separation of parents and children even before World War Two (Bowlby, 1940). Later he turned to observations in connection with separation that called for an explanation. One of the first observation studies was carried out in England and included children under four years of age who had been separated from their parents due to the events of World War Two. The second series of studies, which violated basic ethical guidelines by today’s standards, involved children who had accompanied their mothers to prison during their first year of life. Initially, they were looked after by their mothers, but later they were separated from their mother for three months and cared for by another mother in the same prison. The separation did not involve any change in environment, apart from the separation from the mother. A third series of studies dealt with situations where either the child or the mother was hospitalised. Despite certain differences in background conditions, the results of these studies all pointed in the same direction: From the age of six months, children show a characteristic response when they are separated from their primary caregiver.