Demographic Determinants Of Aging
In its earliest stages, population aging is most influenced by fertility decline rather than by mortality decline, which tends to be concentrated at the youngest ages as the importance of infectious, parasitic diseases declines and that of chronic, degenerative diseases increases. In 1925, Japanese women gave birth to an average of 5.1 children each, but by 1950 fertility had fallen to 3.7 children and by 1960 to below the replacement level of 2.1 children per woman (the number of children needed to just replace a couple in the population). Japan did not experience the long postwar baby boom that the United States did.
In 2000, Japanese women were having on average fewer than 1.4 children each. Although fertility within marriage and ideal family size declined very little in the last two decades of the twentieth century, increases in educational and work opportunities for women in that period were associated with later ages of marriage, and thus with reduced overall fertility. Such low fertility, even combined with lower mortality, means that the absolute size of the Japanese population will likely begin to decline between 2005 and 2010; the size of the cohort of twenty to twenty-four year olds who might be expected to enter the labor force has already begun to decline.
Now that Japan has progressed so far in its demographic transition, mortality decline is the primary force behind aging. Life expectancy at birth increased from about forty-five years in 1925 to sixty years in 1950 and to eighty-one years in 2000, the highest on earth. As in all richer countries, women outlive men; their life expectancies are eighty-four and seventy-seven years, respectively. A major contributor to increased survival in Japan has been the significant decline since 1970 in the death rate from cerebrovascular disease, which until 1980 was the leading cause of death. In the late 1990s, Japan was atypical of richer countries in that cancer was the number one cause of death instead of heart disease.
This rapid increase of survival means that the coming decades will see even greater aging in Japan. In its 1998 update, the United Nations projected that in 2050 the proportion of individuals age sixty-five and over would be 36 percent, assuming that life expectancy increased to eighty-four years and fertility remained at 1.4 children per woman. A fertility rebound to 1.8 children would result in only 32 percent sixty-five and over; an even greater increase to replacement-level fertility would result in 29 percent of the population age sixty-five and over. The proportion of people age eighty and over would increase from less than 4 percent in 2000 to 10, 12, or 13 percent in 2050, depending on the fertility scenario. Thus, the stage is set for continued dramatic aging of the Japanese population. Other countries in Asia, especially those in East and Southeast Asia, will also experience significant aging in the first half of the twenty-first century, but Japan has a substantial head start due to its earlier fertility decline and its remarkable success in reducing mortality.