In nearly all populations throughout the world there are substantially more older women than men. Although there are many biological explanations for why women have greater longevity than men, the higher proportion of women in the older population appears to be a phenomenon of the twentieth century. Using contemporary data on population size and life expectancy in a large number of countries and historical life table data from a diverse subset of countries, cross-national contrasts and historical trends in the female to male ratio are explored. In the 1990's, only 4 countries had fewer women than men in the age group 75 years and older. The number of women per 100 men aged 75+ in the remainder of the world's countries ranged from 100 to 355. In general, countries with a lower overall life expectancy had a lower number of women per 100 men aged 75+, while countries with higher overall life expectancy had a higher female to male ratio in this age group. A hundred years ago there were nearly equal numbers of women and men aged 75+ in many countries. In all countries studied, the female to male ratio increased as the century progressed. Historical life table data were used to calculate the probability of surviving through 5 stages of life: ages 0 to 5, 5 to 15, 15 to 40, 40 to 65, and 65 to 85. Although the probability of survival through all age intervals increased dramatically during the century, the current disparity in the size of the older populations of men and women can be explained primarily by the divergence in male and female probabilities of survival for the two older age intervals as the century progressed. Thus, with higher life expectancy, whether it be comparing countries or over time within a country, the proportion of the older population that is female is greater. Changes in survival probability in middle and late life, rather than childhood and young adulthood, have been responsible for the increased number of women compared to men in the older population.