Critical period: a history of the transition from questions of when, to what, to how.

Although age appears to be the defining characteristic of the concept of critical period, central to its investigation is the recognition that there are specific events which must occur in a particular order for the typical development of certain characteristics to occur. A brief history of some research on critical periods reveals that our questions have shifted from those of: is there a critical period and, if so, when does it occur; to questions of what contributes to the criticality of the period; and finally to how is criticality controlled during development. Abandoning age as a defining component of development has permitted the discovery of exactly how previous and current events construct subsequent events in the process of development. The shifts in questions about critical periods mark an increasing sophistication in understanding how development can be controlled.