Osteoporosis, although a disorder of antiquity, has become more preval
ent in developed countries and is a major risk factor for skeletal fra
cture. Accordingly, the increasing incidence of hip fracture among the
elderly within developed nations has been attributed to an increased
prevalence of osteoporosis. An increasingly sedentary lifestyle has be
en suggested as a significant contributing factor for the increased pr
evalence of osteoporosis. However, differential survival, reflecting c
hanging competing mortality risks, will alter the gene pool of a survi
ving population cohort. Thus, the gene pool (and hence, disease suscep
tibilities) of 70-year-old individuals in 1990, for example, should no
t implicitly be assumed to be the same as 70-year-old individuals in 1
950. Consequently, differences in the prevalence of osteoporosis or in
cidence of hip fracture between current and past elderly cohorts do no
t necessarily imply differences in environmental risk factors such as
levels of physical activity. Instead, variation in competing mortality
risks over time may produce differential survival with selection bias
and ''naturally'' lead to increases in the incidence and prevalence o
f some aging-related disorders such as osteoporosis.