A specified time span, also referred to as the optimal or sensitive period, during which certain events or experiences must occur in order for the development of an organism to proceed normally.
Although this term is used in a variety of contexts, the term is most closely associated with ethology, the study of animal behavior in its natural environment from the perspective of evolutionary adaptation. The critical period plays an important role in the concept of imprinting, first used by Konrad Lorenz in connection with the earliest process of social attachment in young animals. (However, the term imprinting is also applicable to any irreversible behavioral response acquired early in life and normally released by a specific triggering stimulus or situation.) In the most famous example of imprinting, Lorenz demonstrated that exposure to an appropriately maternal object during a critical period would activate the "following" instinct of newborn goslings: he successfully had a group of goslings follow him after he "impersonated" their absent mother.
Other examples of critical periods include the initial four months of life during which puppies must be exposed to humans in order to make good pets and the early months in which birds must be exposed to the characteristic song of their species in order to learn it. Critical periods vary in length: the period for identifying one's mother may last only a few hours, while the period for learning to identify a mate may take several months.
The specifically human phenomenon of language development also appears to be subject to a critical period. So-called "wild" or "feral" children deprived of human society for an extended period show that they have been unable to catch up on language due to lack of exposure early in life.
The term "critical period" is also used to describe physiological as well as behavioral phenomena. For example, the embryonic stage in humans is a critical period for certain types of growth (such as the appearance of the heart, eyes, ears, hands, and feet) which must occur for prenatal development to proceed normally.
Denny, M. Ray. Comparative Psychology: Research in Animal Behavior. New York: Dorset Press, 1970.
Lorenz, Konrad. The Foundation of Ethology. New York: Springer-Verlag, 1981.