Critical/Sensitive Periods

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?