The critical period concept: Research, methodology, and theoretical issues.

The concept of periods in postnatal behavioral development is reviewed. Evidence on the criteria and characteristics of critical period phenomena is considered with respect to endogenous and exogenous influences; methodology of critical period research is described and evaluated; and conceptual issues and problems are discussed in light of past attempts at subclassification of the field and recovery of function as a refutation of the critical period hypothesis. It is concluded that the critical period concept is viable if certain limitations are observed.