Attachment theory stresses the importance of bonding between an infant and a primary caregiver. John Bowlby and Mary Ainsworth were both major players in developing this theory. Ainsworth’s “Strange Situation” provided research on the differences between children’s attachment to their primary caregiver based on how they reacted to being left alone in a room with a stranger. According to Ainsworth, there are four types of attachment: secure, anxious-ambivalent, anxious-avoidant, and disorganized. Any of the non-secure attachment types can result from neglect, abuse, or other type of family/ caregiver issues.
Bretherton, I. (1992). The origins of attachment theory: John Bowlby and Mary Ainsworth. Developmental psychology, 28(5), 759.
Harry Harlow, an ethologist, studied infant-mother attachment in rhesus macaques, finding that infant monkeys preferred a cloth covered “mother” to a wire “mother” with milk. This furthered attachment theory for both humans and animals by suggesting that attachment is more than just providing sustenance and fulfillment of physical needs, but may also play a role in emotional satisfaction and growth.
Harlow, H. F. (1959). Love in infant monkeys. Scientific American, 200(6), 68-75.