Attachment theory describes the dynamics of long-term social relationships between humans. Attachment in infants is primarily a process of proximity-seeking to an identified attachment figure in situations of perceived distress or alarm for the purpose of survival. In other words, infants develop attachment to their caregivers—upon whom they are dependent—as a means of survival. John Bowlby and Mary Ainsworth were two prominent researchers who advanced the theory of attachment as related to human development.

Bowlby's Attachment Theory

John Bowlby's contributions to the theory of attachment formation are heavily influenced by ethology (the scientific study of human and animal behavior), including an emphasis on the evolutionary origins and biological purposes of behavior. According to Bowlby, children are biologically predisposed to develop attachments to caregivers as the result of genetics. In 1969 Bowlby studied mother-infant interactions and concluded that infant smiling, babbling, crying, and cooing are built-in mechanisms to encourage parents to attach to, and thereby care for, the infant. Keeping the parent in close proximity ensures the infant will avoid danger. Bowlby introduced the idea of the caregiver as a "secure base" for the child, and that this secure base was either successfully created during childhood or was not.

The development of parent-infant attachment is a complex process that leads to deeper and deeper attachment as the child ages. This attachment (or lack thereof) has lifelong implications for the child as he or she reaches adulthood. Bowlby conceived of four stages of attachment that begin during infancy: preattachment, attachment-in-the-making, clear-cut attachment, and formation of reciprocal relationships.

• Preattachment (birth to 6 weeks): Built-in signals, such as crying and cooing, bring a newborn baby into close proximity with their caregiver. Babies recognize a caretaker's smell and voice and are comforted by these things. When the caretaker picks up the baby or smiles at her, the beginnings of attachment are forming. However, complete attachment has not yet occurred, so the baby is still comfortable being left with an unfamiliar person.
• Attachment-in-the-making (6 weeks to 8 months): Attachment is getting stronger during this stage, and infants respond differently to familiar people than they do to strangers. For example, a 5-month-old baby will be more "talkative" with his mother rather than with an uncle he sees only once a month. He will also be calmed more quickly by the mother's presence than by the uncle's. Separation anxiety (becoming upset when a trusted caregiver leaves) has not set in yet but will be seen in the next stage. Parents continue to build attachment by meeting the baby's basic needs for food, shelter, and comfort.
• Clear-cut attachment (8 months to 18 months): Attachment to trusted caregivers continues to strengthen in this stage, and separation anxiety is likely in a caregiver's absence. Toddlers generally want to be with their preferred caregiver at all times, and they will follow the caretaker, climb on them, or otherwise do things to keep the caregiver's attention. Parents and other important adults in the child's life continue to strengthen attachment by being receptive to the child's needs for attention, meeting basic needs, and playing with the child.
• Formation of reciprocal attachment (18 months to 2 years): Rapid language growth facilitates the understanding of new concepts, and children begin to understand a parent's coming and going. For example, children can now understand that a parent returns home from work at a certain time each day, so separation anxiety lessens—although the child may do things to gain extra time with the parent prior to departure or to keep the parent from leaving. Parents can help a child form secure attachment by explaining things to them, by being present as much as possible, and by continuing to meet basic needs.

Ainsworth's Types of Attachment

In 1970, Mary Ainsworth built on and expanded Bowlby's ideas, coming up with a more nuanced view of multiple types of insecure attachment. Ainsworth's primary contribution to attachment theory comes in the form of a study known as the Strange Situation. In this study, Ainsworth placed children between the ages of 1 and 2 in unfamiliar situations to assess the type and level of their attachment to their caregivers. Her research showed that children generally use the parent as a secure base from which to explore an unfamiliar room, and they become upset or uncomfortable when the parent leaves and a new individual (not known to the child) enters the room.

Ainsworth identified four primary types of attachment: secure, avoidant, and resistant/ambivalent. Depending upon how the children attached to their parents, they would act in predictable ways in the Strange Situation experiment. 

• Secure: Children with this form of attachment use the caregiver as a secure base from which to explore the room. They are comforted by the parent and show a clear preference for the caregiver (for example, by protesting or avoiding the unfamiliar person).
• Avoidant: These children avoid contact with the caregiver and show little interest in play. They do not seem to mind when the caregiver leaves, and they treat the stranger in a similar fashion to the caregiver. The child may act in a rebellious manner and have lower self-esteem as they get older. The children of parents who do not meet their basic needs or are inattentive may form avoidant attachment.
• Resistant/Ambivalent: Children with this form of attachment are unable to use the caregiver as a secure base, and they seek out the caregiver prior to separation. They are both distressed by the caregiver's departure and angry when the caregiver returns. They are not easily calmed by the caregiver or the stranger, and they feel anxiety with the caregiver due to inconsistent attachment patterns.

In 1990, a fourth category, known as disorganized attachment, was added by Ainsworth's colleague Mary Main. Children with these attachment patterns engage in stereotypical behavior such as freezing or rocking. They act strangely with the caregiver and do not appear to know how to attach, doing such things as approaching with their back turned or hugging the stranger upon their entry to the room. Disorganized attachment generally results from the child being maltreated or neglected in some way.

Childhood Attachment

Children who have secure attachment to parents are more likely to be successful adults.

Harry Harlow and the Rhesus Monkeys

In order to demonstrate the importance of social and emotional development in people, Harry Harlow studied the attachment patterns of Rhesus monkeys. This was based on the belief of John Bowlby that maternal attachment is a necessity for proper emotional and social development. Harlow raised baby Rhesus monkeys in a nursery-type setting away from their mothers; he gave them surrogate mothers made out of wire and wood, to which the babies developed attachment bonds. His alternative rearing technique, also called maternal deprivation, is considered highly controversial today. 

Harlow next chose to investigate if the baby monkeys had a preference for bare wire mothers or cloth-covered mothers. For this experiment, he presented the infants with a cloth mother or a wire mother under two conditions. In one situation, the wire mother held a bottle with food and the cloth mother held no food; in the other, the cloth mother held the bottle and the wire mother had nothing. In the end, even in the situations in which the wire mother had food and the cloth mother had none, the baby monkeys preferred to cling to the cloth mother for comfort. Harlow concluded that there was much more to the mother/infant relationship than milk, and that this "contact comfort" was essential to the psychological development and health of infants.