The concept of sensitive periods is useful in forming an idea of 'biological expectation'. Some examples pertaining to physical and metabolic insults provide clear-cut examples of critical period development in which early events have clear effects later in childhood and adolescence. Many more supportive observations are found in the developmental neurobiology literature. While these are clear examples of the sensitive period concept, application of the same set of principles and definitions to psychosocial development is not so straightforward. The operationalization of sensitive periods in psychosocial research is confounded at the outset since the object of measurement is an open-ended system of adaptability, as later experience may modify or reverse the constraints imposed by early experiences, which seems the opposite of the sensitive period concept. Research indicates the difficulty in determining the onset or cessation of sensitive period intervals for factors influencing psychosocial development. Indeed, the dependence of adaptation upon the overlapping development of multiple neural systems is readily apparent.Emotional regulation, insecure attachment, and behavioral problems that emerge in early childhood appear to be maintained, or modified, along with the path to adolescence through ongoing transactions within primary family relationships that facilitate or prevent adaptive retranscription of the developing individual's internal working model. Emotional regulation and autonomy come to be mastered through the contingency of parental responsiveness and in particular the cogent expression of affective meaning and value. Simultaneous consideration of evidence from physical, physiological and psychological levels of explanation provides insights into the interactional nature of nurture. The layering of information in the manner presented also provides an idea of the limitations of the sensitive period concept while illustrating the practical domains where appropriate influence optimize the opportunity for optimal development. May you find this book engaging and edifying.