Attachment and Health
John Bowlby and Mary Ainsworth developed attachment theory in the 1960s while investigating the effects of maternal separation on infant development. The development of the Strange Situation task in 1965 by Ainsworth and Wittig allowed researchers to systematically investigate the attachment system operating between children and their parents. The Strange Situation involves separating infant-parent dyads and observing infant behaviours upon reunion. Infant behaviour tended to follow patterns, leading to the development of three attachment categories: Secure, Anxious-Avoidant, and Anxious-Resistant. In 1990, Ainsworth added a new category, championed by Mary Main, called Disorganized/Disoriented attachment, characterized by its less reliable pattern of behaviour.
Object relations theory
Attachment theory, researched by John Bowlby and others, has continued to deepen our understanding of early object relationships. While a different strain of psychoanalytic theory and research, the findings in attachment studies have continued to support the validity of the developmental progressions described in object relations. Recent decades in developmental psychological research, for example on the onset of a "theory of mind" in children, has suggested that the formation of the mental world is enabled by the infant-parent interpersonal interaction which was the main thesis of British object-relations tradition (e.g. Fairbairn, 1952).