The Strange Situation procedure was formulated to observe attachment relationships between a caregiver and children between the age of nine and 18 months. It was developed by Mary Ainsworth, a developmental psychologist Originally it was devised to enable children to be classified into the attachment styles known as "secure", "anxious-avoidant" and "anxious-ambivalent". As research accumulated and atypical patterns of attachment became more apparent it was further developed by Main and Solomon in 1986 and 1990 to include the new category of disorganized/disoriented attachment.