Critical-Period Hypothesis

History and Claims

The sensitive-period hypothesis was first developed in experimental embryology. In the 1920s, Charles Stockard (1907, 1921) found that specific birth defects occurred in fish when normal development was disturbed by exposure to extreme temperatures and toxic chemicals during a particular stage as a specific embryonic organ was developing. Similarly, Hans Spemann (1938) discovered that at a particular time in their ontogeny, certain cells transplanted from one site in the embryo to another site matured like cells at the new site, but at other times, the cells developed as they would have at their original site in the embryo. On this basis, the idea was advanced that the various parts of the whole organism (e.g., various organs or organ systems) emerge in a fixed sequence, with a limited amount of time allowed for each part to develop. Any part that does not develop normally or appropriately during its own critical period will not have another chance. The focus of development will shift to another organ system in accordance with the predetermined timetable of development, which will then be in its critical period.

Konrad Lorenz (1937) introduced the notion of the critical period into studies of animal and, eventually, human behavior. In his work on imprinting in birds, Lorenz observed that hatchlings of certain bird species fixated, or imprinted, on the first large moving object they saw, and then followed that object around as they would normally follow their mothers. Within a few days of hatching, though, this tendency to imprint ceased to exist. Lorenz found that the features of imprinting were analogous to features Spemann had identified in embryological development, such that environmental influences had effects confined to limited periods of development, and in both cases, the results were irreversible. Lorenz's studies of imprinting in birds gave rise to research in a variety of animal behaviors, ranging from sociability in dogs to emotional development in monkeys. Others extended these ideas to human development, including sensitive periods in human socioemotional development, the establishment of secure attachments between infant and caregiver, development of the visual system, and the acquisition of language.

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