What Do We Know about Sensitive Periods in Human Development and How Do We Know It?

Abstract

For many years, developmental psychologists have argued for the importance of early experiences with regard to subsequent adaptive behavioral development. Along with this argument has been the corollary that there are critical or sensitive periods during which certain types of experiences must happen for typical development to occur. But a quick inspection of the evidence for both the importance of early experience and the nature of sensitive periods would suggest that most of the data supporting these two tenets are from animal or rodent studies. That is not to say that the research literature on human infants and children is devoid of studies of the effects of negative experiences such as stress, poverty, and maltreatment. These studies abound. But as a discipline, and because of clear ethical considerations, we must rely on research that is correlational in design to understand early experience and sensitiveperiod effects. These studies on the effects of adverse experience are compelling but even more so are the animal and rodent studies where random assignment and experimental manipulation are more easily created with data suggesting that early experience and its timing are of significant importance for typical development. The studies in rodents by Victor Denenberg, Seymour Levine, and, more recently, Paul Plotsky and Michael Meaney are examples of this work, as is the work with nonhuman primates by Harry Harlow and his students. The translation of this work has been more widely accepted recently with the advent of advances in both brain imaging methods and genetic and epigenetic approaches to understanding the underlying neural mechanisms supporting development. But there is still a dearth of experimental evidence with human subjects that can support the animal work. Of course, there are exceptions, particularly in the areas of sensation and perception. Following up on the animal work of Hubel and Wiesel, who identified the importance of early visual experience for the formation of cortical architecture involved in vision, Daphne Mauer, in a series of studies, demonstrated the importance of this experience and the nature of the timing of experience with infants who had been born with cataracts and who had surgery at different points in the first year of life, thus providing them with patterned visual stimulation at different points in development. There is also a good deal of work on a phenomenon known as perceptual narrowing. Human infants appear capable of discriminating the sounds from languages from