Strange situation
The strange situation is a procedure devised by Mary Ainsworth in the 1970s to observe attachment in children, that is relationships between a caregiver and child. It applies to children between the ages of 9 and 30 months. Broadly speaking, the attachment styles were (1) secure and (2) insecure (ambivalent and avoidance). Later, Mary Main and her husband Erik Hesse introduced the 4th category, disorganized. The procedure played an important role in the development of attachment theory.