The Age of Puberty Determines Sexual Dimorphism in Bone Structure: A Male/Female Co-Twin Control Study

Background: Taller stature and larger bone size in males are attributed to more rapid growth than in females. However, comparing sexes of the same age mismatches by pubertal stage, so males will be less mature than females. Comparing sexes of the same pubertal stage mismatches by age, so males will be older than females. Objective: We hypothesized that sex differences in stature and bone structure are the result of sex differences in the duration but not the rate of prepubertal and pubertal growth. Methods: We measured bone dimensions in 90 male/female co-twin pairs aged 7–18 yr using anthropometry and dual x-ray absorptiometry. Forty-two pairs had follow-up assessments. Within-pair differences were expressed as a percentage of the pair mean. Results: Thirty percent of the 1–1.5 sd sex difference in bone widths and midfemur bending strength observed in 11 postpubertal pairs was present in 43 prepubertal pairs. In prepubertal pairs, annual growth in leg length was about 1.5 times truncal growth, but neither rate differed by sex. During puberty, truncal growth in both sexes was higher than before puberty but did not differ by sex. The longer period of pre- and intrapubertal growth in males produced most of the sex difference in bone morphology observed in postpubertal twins. Conclusion: Sex differences in bone morphology are the result of the later onset of puberty in males, not more rapid growth. Differences in bone widths are partly established before puberty. Larger bone size in males results from a longer growth period, not greater growth velocity, and greater bone widths in males are partly established before puberty.