ABSTRACT
THEODORE D. WACHS, MICHAEL GEORGIEFF, SARAH CUSICK, AND BRUCE S. MCEWEN
15.1 INTRODUCTION
Evidence documents that children from low-income families in both wealthy[1] and low-income countries[2] have greater exposure to multiple biological and psychosocial risks that can significantly compromise their development. These findings emphasize the importance of integrating and implementing multidimensional biological and psychosocial interventions to compensate for exposure to multiple risks.[3] A critical and long-standing question involves identifying the age period(s) in which such interventions can have the strongest and longest lasting effects. The
concept that the early years of life are a time when children are particularly sensitive to extrinsic influences has deep-seated roots, dating back to the writings of Plato.[4] In the present era, questions involving timing of events and change over time in relations between contextual elements are central issues in major developmental theories such as developmental systems theory[5] and the bioecological model.[6]
Initial empirical support for the importance of the early years of life came from 20th century embryological research on fetal development and ethological research on imprinting, which culminated in the concepts of critical and sensitive periods of development. While both concepts refer to age periods characterized by plasticity in development, when the effects of exposure to facilitative experiences or developmental risks are particularly strong and lasting, the concepts are not identical.[7] Critical periods are characterized by enhanced sensitivity to exposures that are restricted to a sharply defi ned time period such that the effects of exposures during this time period are irreversible. In contrast, when sensitive periods occur, the exposure time windows for enhanced sensitivity are broader, and there can be continued, though reduced, plasticity both before and after the sensitive period and exposure during sensitive time windows is not necessarily irreversible.[8]
Evidence from human-level studies favors the operation of sensitive rather than critical periods.[9-12] Research fi ndings also indicate that there may be multiple sensitive periods depending upon the domains of development assessed.[9, 10, 13] Illustrating the operation of multiple sensitive periods is evidence that sensitive periods for neural development may be narrower than sensitive periods for behavioral development[14, 15] and that different sensitive-period windows are seen for cognitive/academic versus social-emotional outcomes.[16, 17] For example, the impact of exposure to poverty (or to interventions designed to reduce poverty) upon later cognitive or academic outcomes appears to be strongest in the period from infancy to early childhood, whereas such exposure appears to adversely affect social-emotional development or behavior problem outcomes across the age span from infancy through adolescence.[18] One implication of this pattern of fi ndings is that different time periods may be needed for biological versus psychosocial interventions or for different psychosocial outcomes.
