Calendar year

Figure 2 Mean or median age of menarche as a function of calendar year from 1790 to 1980. The symbols refer to England 0; France •; Germany Holland □; Scandinavia (Denmark, Finland, Norway, and Sweden) X; Belgium, Czechoslovakia, Hungary, Italy, Poland (rural), Romania (urban and rural), Russia (15.2years at an altitude of 2500m and 14.4years at 700m), Spain, and Switzerland, all labeled O; and the United States 0 (data not included in the regression line). Twenty-seven points were identical and do not appear on the graph. The regression line cannot be extended indefinitely. The age of menarche has already leveled off in some European countries, as it has in the United States. (From Wyshak and Frisch (1982) Evidence for a secular trend in age of menarche. New England Journal of Medicine 306: 1033-1035, with permission from the New England Journal of Medicine.)

directly related to the quantity of circulating estrogen. The biological effectiveness of the estrogen is also related to body weight. Rate of fat gain, therefore, is a neat mechanism for relating rate of growth, nutrition, and physical work to the energy requirements for reproduction.

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