Do sex differences in brain structure or function during adolescence explain sex differences in functional capacity and behavior? Answering this question is one of the next pressing tasks for better understanding how the processes of sexual differentiation affect behavior or risk for psychopathology. Separating out effects of development from those of sex is challenging, particularly in cross-sectional data, given the enormous variability within the normal range of both brain structural features and ability. For example, if brain volumes in the frontal lobe appear to peak two years earlier in females, suggesting more rapid development in females, is matching populations on chronological age the most appropriate method, or should developmentally equivalent groups be chosen? And if the latter, what measure of development should be used? Pubertal stage is one possibility, as seen in some of the studies here, but the timing of pubertal maturation relative to other aspects of adolescence varies between individuals. Moreover, different systems may not mature at the same rate even within the same person, and not all cognitive maturation is
