The reasons for this secular trend are not well understood. The reduction of such growth-retarding factors as poor nutrition and chronic illness has been cited most often, and recently documented trends toward a later age of menarche in Bangladesh appear to support this hypothesis. Whether this historical acceleration in the rate of general body maturation has been associated with comparable changes in the rate of brain development and level of performance of children growing up in industrialized societies is unknown. Preliminary data do suggest, however, that variations in age of onset of puberty may have developmental and behavioral consequences during adolescence. In a study of more than 5,000 white males and females ages 12-17 ,late-maturing males scored lower on education-related variables and early-maturing males scored higher than those in the middle maturity groups. These differences were found to be independent of measured intelligence and weaker at age 12 than at older ages. No consistent and statistically significant differences related to maturational status were found among females. Other studies, reporting inconsistent findings that imply disadvantages for early-maturing girls as well as late-maturing boys, suggest that cultural context and social class may be important determinants of the influences on behavior of the differential timing of physical maturation. Interactions among academic achievement, personal and social development, and physiologic maturity are not well understood. Further studies of these relationships, particularly regarding the early onset of pubertal changes in the middle childhood years, are needed.