The growing participation of American women in the labor force has been, in Marshall and Paulin's words, "perhaps the most important labor market development of the century." In fact, most of their movement into the labor force has occurred in the past half-century. In 1900, one woman in five was employed, and over the next 40 years, the percentage of women in the labor force increased only six percent, or less than two percent per decade. In 1940 just over one-fourth of U.S. women participated in the labor force. They made more dramatic gains in the next decade. During World War II over one-third of adult women were employed outside the home, and after the war many did not leave the workforce. By 1950, 31% were in the labor force, and by 1960, 35%. In the 1960s there was striking growth, almost one percent per year, such that by 1970, 43% of women were in the labor force. The trend since 1970 has been consistent annual increase at a lower rate. By 1980 the labor force participation rate of U.S. women stood at 51%, and in 1988 it was 56%. With respect to marital status, it has always been the single women who were most likely to work. In 1950, half of all single women were in the labor force; by 1986, two-thirds were. Until the late 1960s, divorced and widowed women participated more in the labor force than the married women, but in 1966 the rate for married women surpassed that of the widowed and divorced, and since then labor force participation of married women has continued to increase faster than that for the widowed and divorced. The percentage of all married women in the labor force increased to 41% in 1970, 51% in 1980, and 57% in 1988. The corresponding rates for widowed and divorced women were 36%, 41%, and 43%. Thus, while both single women and the widowed and divorced have also increased their labor force participation, the sharpest gains have been among married women, whose participation rates rose from 25% in 1950 to 57% in 1988.