Critical/Sensitive Periods

Illustrative examples of the concept of a critical/sensitive period can also be found in the domain of social development. One particularly interesting example is the formation of the infant-parent attachment relationship.

Attachment is the strong emotional ties between the infant and the caregiver. This reciprocal relationship develops over the first year of the child's life, and especially during the second six months of the first year. During this time, the infant's social behavior becomes increasingly organized around the principal caregiver.

John Bowlby, a twentieth-century English psychiatrist who was strongly influenced by evolutionary theory, formulated and presented a comprehensive theory of attachment. In the late 1950s and early 1960s, he first proposed that there is a strong biological basis for the development of this relationship. According to Bowlby, the infant-parent attachment relationship develops because it is important to the survival of the infant and also provides a secure base from which the infant can feel safe exploring their environment.

Bowlby suggested that there was a sensitive period for the formation of the attachment relationship. This period is from approximately six months to twenty-four months of age and coincides with the infant's increasing tendency to approach familiar caregivers and to be wary of unfamiliar adults. In addition, according to Bowlby and his colleague Mary Ainsworth, the quality of this attachment relationship is strongly influenced by experiences and repeated interactions between the infant and the caregiver. In particular, Ainsworth's research, that was first published in the late 1960s, demonstrated that a secure attachment relationship is associated with the quality of caregiving that the infant receives. More specifically, consistent and responsive caregiving is associated with the formation of a secure attachment relationship.

If the period from six months to twenty-four months is viewed as a critical period for the development of the attachment relationship, the relationship must be formed during this specific period in early development. Alternatively, if this period is viewed as a sensitive period, the infant-parent attachment relationship will develop more readily during this period. After the sensitive period, this first attachment relationship can develop, but with greater difficulty. As in the case of language development, information about whether there is a critical or sensitive period for the formation of a secure attachment relationship comes from different sources. These sources include cases of infants who did not experience consistent caregiving because they were raised in institutions prior to being adopted.

The early research documenting such cases was published in the 1940s. This research consistently reported that children reared in orphanages for the first years of life subsequently exhibited unusual and maladaptive patterns of social behavior, difficulty in forming close relationships, and indiscriminately friendly behavior toward unfamiliar adults. The results of this early research contributed to the decline of such forms of institutional care. Furthermore, these results supported the notion of a critical period for the formation of the attachment relationship.

Research published in the 1990s has contributed to a modification of this notion of a critical period. These research results have come from studies of infants in Eastern Europe who were abandoned or orphaned and, therefore, raised in institutions prior to adoption by families in North America and the United Kingdom. These results have indicated that these adoptees were able to form attachment relationships after the first year of life and also made notable developmental progress following adoption. As a group, however, these children appeared to be at an increased risk for insecure or maladaptive attachment relationships with their adopted parents. This evidence, then, is consistent with the notion of a sensitive period, rather than a critical period, for the development of the first attachment relationship, rather than a critical one.