Psychologist John Bowlby’s seminal work on attachment postulated that attachment of infants to their caregiver(s) is an evolutionary process that plays a vital role in the wellbeing of children. Mary Ainsworth, who worked alongside Bowlby, conducted the Strange Situation Test, whereby children were left alone in an unfamiliar environment and then reunited with their primary caregiver. The researchers noticed three attachment styles in the infants: secure, avoidant, and ambivalent.

Available and responsive caregivers create secure attachment in children; securely attached children will seek their parent(s) for reassurance when in a strange place and will be soothed by their presence. On the other hand, consistently unavailable or unattuned parents (parents who are not attentive to or aware of the physical or emotional needs of their child) will create children who are avoidantly or ambivalently attached; these children do not reliably seek their parents for comfort—they either avoid them, seem indifferent to their presence, or are unable to be soothed by them—as their parents have not reliably provided care in a way that the infant can perceive.

Harry Harlow built on Bowlby’s attachment research in his famous experiment on infant Rhesus monkeys who were separated from their mothers. The infants were housed with two wire monkeys; one held a milk bottle, and the other was covered with soft cloth.