The concept of critical/sensitive periods is of interest in discussions of the influence of biological and experiential factors during periods of developmental change. A critical or sensitive period is defined as a period when certain experiences are particularly important because they have a significant influence on later development. Let us begin our consideration of this concept with a case example that illustrates some of the significant developmental changes that occur during the period of infancy and early childhood.
Eve is a typical, healthy newborn human infant. She is raised by parents who are sensitive to her unique needs and characteristics, and who regularly provide her with appropriate stimulation and parenting. The first five years of Eve's life will be witness to rapid, significant changes in her behavior and abilities.
Language development provides one example of these changes. As a newborn, Eve prefers the sound of human voices to other sounds and can discriminate between the language of her culture and other languages. She cries, usually when she has some physical needs, such as hunger. By five to six months of age, she will coo when contented and may be babbling or producing simple combinations of consonants and vowels. By twelve to eighteen months of age, she will be speaking in single word sentences and will understand more than fifty words. By twenty-four months of age her vocabulary will have expanded to approximately 200 words and she will be producing hundreds of different two- and three-word sentences. By the age of five, Eve will have a vocabulary of about 2,000 words and will use many of the grammatical structures of her native language, without ever taking part in a formal language lesson.
The period from birth to five years of age will also include significant changes in Eve's social relationships. As a newborn, she can discriminate her mother's face, voice, and smell from all others. By three months of age, she will smile at her parents and will react positively to most strangers. Between seven and twelve months of age she will begin to demonstrate specific attachments to her parents, will display anxiety when strangers approach, and will be distressed by separation from her parents. By three to four years of age, Eve will continue to be securely attached to her parents, but her distress at separation from them will diminish, and she will be able to confidently participate in a nursery school program.
The study of human development is the study of change. As the case of Eve illustrates, the changes that occur during infancy and childhood are happening at a pace that is more rapid and impressive than at any other period in the lifespan. Although psychologists agree that these developmental changes during infancy and childhood are impressive and extraordinary, they often disagree on the best way to understand and explain these changes.
One area of disagreement is the discussion of whether developmental changes are the result of biological, genetic factors or of the kinds of experiences that the child has had. Another area of disagreement is the discussion of whether developmental changes occur in a series of unique stages or periods.
The concept of a critical/sensitive period is related to both of these areas of discussion. The critical/sensitive period is determined by biological maturation and characterized by increased vulnerability or responsivity to specific experiences. If these specific experiences occur during this period, then development will continue on its typical course. If these specific experiences do not occur, there may be a significant disruption or difficulty in subsequent development.
What evidence is there supporting this notion of a critical period in development? Is there a difference between a critical period and a sensitive period?
Language Development: Critical or Sensitive Period?
Language development provides one example that can illustrate the concept of a critical/sensitive period. Although language development is a process that psychologists have long debated, there is agreement that a strong biological basis for language acquisition exists. In 1967, Eric Lenneberg first proposed the notion of a critical period for language acquisition. He suggested that the period between infancy and puberty (the beginning of adolescence) was a critical period for language acquisition. This critical period was thought to end at puberty because of important maturational changes in the brain that occur at this time. Language must be acquired during the critical period if it is to be acquired at all. Alternatively, if the period from infancy to puberty is viewed as a sensitive period, rather than a critical period, language will be learned most easily during this period. After the sensitive period, language can be learned, but with greater difficulty and less efficiency.
What evidence is there supporting this concept of a critical period for language acquisition? How could a researcher ever test this idea? Information about this issue comes from different sources. These sources include a few unfortunate and extreme cases of childhood deprivation—children who were deprived of typical social experiences and stimulation.
Perhaps the most well-known of these was the case of Genie, who was described in a series of publications in the 1970s. Genie was essentially kept in isolation by a maltreating parent, with no exposure to language and normal social experiences for the time between toddlerhood and early adolescence. When she was discovered, she had some understanding of language but did not speak. After nearly one year of intensive training and instruction, she had a vocabulary of about 200 words and was speaking in two-word sentences. Six years later, she had made much progress, but she was still much less advanced in her language than other people her age who had normal experiences growing up.
Since Genie was able to acquire language following the onset of adolescence, the notion that language learning is impossible after the critical period cannot be supported. Rather, language learning can occur after the onset of adolescence but may be incomplete. The period between infancy and adolescence, therefore, may be a sensitive period for language learning—language can be acquired more easily during this time—rather than an absolutely critical period.
Infant-Parent Attachment: A Critical/Sensitive Period for Social Development
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.
Curtiss, Susan. Genie: A Psycholinguistic Study of a Modern Day "Wild Child." New York: Academic Press, 1977.
Marvin, Robert S., and Preston A. Britner. "Normative Development: The Ontogeny of Attachment." In Jude Cassidy and Phillip R. Shaver eds., Handbook of Attachment: Theory, Research, and Clinical Applications. New York: Guilford Press, 1999.
Newport, Elissa L. "Contrasting Conceptions of the Critical Period for Language." In Susan Carey and Rochel Gelman eds., The Epigenesis of Mind: Essays on Biology and Cognition. Hillsdale, NJ: Erlbaum, 1991.
Rutter, Michael. "A Fresh Look at 'Maternal Deprivation."' In Patrick Bateson ed., The Development and Integration of Behaviour: Essays in Honor of Robert Hinde. New York: Cambridge University Press, 1991.