In 1946, Bowlby joined the staff of the famous Tavistock Clinic in London, where he met a social worker named James Robertson, who was angry at the hospital's policy of not allowing mothers to visit young children who'd been admitted for surgery—a hospital policy intended to prevent infections from spreading through the ward. Robertson noticed that children in the second year, but not those who were younger or older, became unusually distressed when they were alone on the ward lying in a crib, and he told Bowlby about his observations. Bowlby interpreted these children's distress as further confirmation of his ideas on attachment. However, Robertson disagreed, arguing that the children's distress could be avoided by simply having any adult present on the ward. This fact meant that the crying didn't reflect the fact that the children missed their mothers, but rather that they were frightened because they were alone in an unfamiliar place.
Several years later, Bowlby read about American psychologist Harry Harlow's research with infant monkeys. Harlow had discovered that infant rhesus monkeys given a choice between resting on a wire object that had a bottle with food or one covered with soft terrycloth and no food spent most of the time on the terrycloth surrogate. More important, they were less frightened by unfamiliar objects when they were resting on the cloth-covered surrogate. Harlow concluded that tactile contact was more critical than food in establishing an emotional bond between the infant and a surrogate, and that this bond awarded the infant a measure of security.
Bowlby visited Harlow and later invited him to conferences. In addition, Bowlby began to read the research in ethology suggesting that animals inherit a special sensitivity to a small set of stimuli called releasers that evoke particular behaviors. Borrowing this idea, he applied it to the mother–infant attachment by arguing that, in human infants, the face, hair, and hands of the mother acted as releasers for the infant's behaviors of grasping and clinging, and her voice and caresses released the infant's smiling and babbling. The infant's display of these responses day after day established an attachment bond between the infant and mother. These were brilliant inferences based on Harlow's research and the evidence from the ethologists, and, in my view, are correct. But, as I noted, we have to separate the fact that infants establish different types of attachment from the controversial belief that the psychological states associated with these attachments are preserved indefinitely.
A pivotal event in the history of attachment theory occurred in 1950, when the Canadian psychologist Mary Ainsworth joined Bowlby's staff at the Tavistock Clinic. Four years later, she went to Uganda because her husband had been sent there, and, with time to spare, she studied Ugandan mother–infant interactions in the natural setting of the home. In 1969, she published Infancy in Uganda, summarizing her observations. In this book, she noted that important, genetically based differences among infants influenced their level of distress to many experiences, not just separation. She concluded that it wasn't possible to separate the contributions to the quality of attachment of the infant's daily experiences from the contributions of his or her biologically based temperament. Unfortunately, she forgot this conclusion when she later joined the faculty of The Johns Hopkins University in Baltimore, where she conducted her famous study of attachment on 23 white, middle-class, mother–infant pairs who'd been observed at home many times in the first year and tested in the Strange Situation.
In this work, the 23 infants who'd been observed at home with the mother during the first year by Ainsworth's students were seen in the Strange Situation at 1 year of age. This procedure consists of a series of 3-minute episodes that last less than 30 minutes in all. In the first episode, the mother and infant are alone in an unfamiliar room containing some toys. Next a stranger enters the room. Then the mother leaves the room, so that the infant is alone with the stranger. A few minutes later, the mother returns to the room, and the stranger leaves. Minutes later, the mother departs again, leaving the child totally alone in the room. The stranger then returns to the room. In the final episode, the mother returns to the child. The critical information used to decide the quality of the infant's attachment is the child's behaviors during the two occasions when the mother prepares to leave the room and when she returns.