Ainsworth spent some years in the early 1950s with John Bowlby at the Tavistock Clinic, London, mainly working on the research into the effects of 'maternal deprivation' (the lack of an adequate mother experience in infancy) on children's development. It was the results of this research that led Bowlby to believe he had found the main reason for juvenile delinquency – inadequate or non-existent 'mothering'.
In 1954, Mary Ainsworth left for Africa, and moved attachment theory forward through her observations of 28 mothers and their children in Uganda. She noted that, although there were some important differences in how children behaved when they were separated from their mothers, it was during reunions after separations that differences between children's behaviour were most evident. She kept in touch with Bowlby, and reported that she had identified three different types of attachment: secure, insecure and absent.
Ainsworth later moved to Baltimore, USA, and spent a long period closely observing and recording the behaviour of 15 infants and their mothers. It was during this time that she clarified her attachment categories by subdividing the insecure classification into two. She also developed a standard method for assessing attachment in infants aged around one year, the Strange Situation, which has become a 'gold standard' laboratory technique for attachment researchers.