What is attachment theory and what did Ainsworth's Strange Situation show?

Attachment theory, developed by John Bowlby, proposes that infants form emotional bonds with caregivers that influence social and emotional development throughout life. Mary Ainsworth tested attachment styles using the Strange Situation procedure, observing infant reactions to caregiver separation and reunion. Secure attachment, seen in about 60 percent of infants, shows distress at separation and comfort upon reunion. Anxious-ambivalent attachment shows intense distress and difficulty being comforted. Avoidant attachment shows little distress and ignores the caregiver upon return.