International variability of ages at menarche and menopause: patterns and main determinants.
The purpose of this study was to review published studies on the variability of age at menarche and age at menopause throughout the world, and to identify the main causes for age variation in the timing of these events. We first present a summary table including mean (or median) values of the age at menarche in 67 countries, and of the age at menopause in 26 countries. General linear models showed that mean age at menarche was strongly linked to the mean female life expectancy, suggesting that one or several variables responsible for inequalities in longevity similarly influenced the onset of menarche. A closer examination of the data revealed that among several variables reflecting living conditions, the factors best explaining the variation in age at menarche were adult illiteracy rate and vegetable calorie consumption. Because adult illiteracy rate has some correlation with the age at which children are involved in physical activities that can be detrimental in terms of energy expenditure, our results suggest that age at menarche reflects more a trend in energy balance than merely nutritional status. In addition, we found the main determinant of age at menopause to be the mean fertility. This study thus suggests that, on a large scale, age at menarche is mainly determined by extrinsic factors such as living conditions, while age at menopause seems to be mainly influenced by intrinsic factors such as the reproductive history of individuals. Finally, these findings suggest that human patterns cannot be addressed solely by traditional, small-scale investigations on single populations. Rather, complementary research on a larger scale, such as this study, may be more appropriate in defining some interesting applications to the practical problems of human ecology.