ABSTRACT
Scientists who study psychological structures or functions from all but an entirely static perspective, and in particular those who are interested in origins and development, inevitably confront the truism that their phenomena are shaped by both endogenous and exogenous forces interacting through time. Moreover, it is now commonplace to find phases in the development of many different psychological structures and functions that are unique in the evolving transaction between these two life forces. At certain times in their life cycles, many structures and functions become particularly susceptible to specific experience (or to the absence of that experience) in a way that then alters some future instantiation of that structure or function. So, during such sensitive periods in development specific experiences dramatically influence eventual outcomes.
