Mary Dinsmore Salter Ainsworth conducted research in the United States, the United Kingdom, and Uganda, in the mid-twentieth century, on how emotional attachment between mothers and infants impacts development. Together with researcher John Bowlby, she developed the Ainsworth-Bowlby Theory of Attachment, hereafter ABTA. The ABTA is a theoretical framework that accounts for how and why attachment and separation, particularly between children and mothers, impact psychological and behavioral development. Using her training in clinical psychological assessments, she developed new techniques for assessing security and attachment between infants and mothers. Ainsworth and her colleagues applied those techniques to several long-term studies that provided empirical support for the ABTA. Many of her assessment techniques are still in use as of 2025, and her contributions to the ABTA have enabled researchers to systematically analyze how attachment, security, and separation impact psychological and behavioral development throughout the lifespan, opening new avenues for research and treatment.