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Two hypotheses relating menarcheal age to physical growth are compared. One suggests that menarcheal age is closely coordinated with skeletal development, the other that it is contingent upon accumulation of sufficient fat. A factor analysis of longitudinal growth data is used to achieve a heuristic separation of weight and height variables. Subsequent regression of menarcheal age on orthogonal factors yields support for both views, but indicates that skeletal maturation exerts a much stronger influence, accounting for three times as much menarcheal age variance as weight or relative weight. This synchronization is further demonstrated by data on the age at which appropriate height and pelvic size for reproduction are attained. The physical and mechanical constraints of reproduction are considered to have effected this synchronization through the agency of natural selection.