Bone size is greater in young men than in women but the load imposed is also greater. Therefore, the load per unit area (stress) on the vertebral body is the same in men and women. Fractures are uncommon in young adulthood because volumetric bone mineral density (vBMD) is normal so that the stress imposed on the bone is well below its strength in both sexes. The gender difference in bone fragility emerges during aging because the biomechanical relationship between the stress on bone and its strength changes in men and women. Periosteal apposition occurs in both genders during aging but more in men than in women so that vertebral cross-sectional area (CSA) increases more in men than women. Consequently, the stress on bone decreases more in men than women. In addition, periosteal apposition offsets endosteal bone loss more in men than in women so that net bone loss is less in men than women. Thus, the stress on the bone decreases more and the strength of the bone decreases less in men than in women. The ratio of the stress on bone to its strength can be quantitated and is below unity in young men and women. During aging this ratio increases to approach or exceed unity, the value at which fracture risk is high because the stress on bone is similar to its strength. As the stress decreases more and strength decreases less in men, the ratio increases less in men than in women so that in old age only 2-3% of men but 20% of women have this stress to strength ratio exceeding unity. Men and women with spine fractures have the same vBMD and same stress to strength ratio which is no different from unity. Fewer men than women are at risk for fracture because fewer men than women have these structural determinants of bone strength below a level at which the loads exceed the bone's ability to tolerate them.