The annual population growth rate in 1960 was 2.7 percent, rising to a high of 3.6 percent in 1970, and falling to 1.9 percent in 1975. This fall reflected a dramatic decline in the fertility rate: the average number of children born to women decreased from 6.5 in 1966 to 2.5 in 1988. Assuming the data is reliable, reasons for falling growth rates and fertility rates probably include late marriage, urbanization, limited housing space, and the expectation that women would participate equally in work hours in the labor force. The experience of other socialist countries suggests that widespread labor force participation by women often goes hand-in-hand with more traditional role expectations; in other words, they are still responsible for housework and childrearing. The high percentage of males age 17 to 26 may have contributed to the low fertility rate. According to Eberstadt and Banister's data, the annual population growth rate in 1991 was 1.9 percent. However, the CIA World Fact Book estimated that North Korea's annual population growth rate was 1.0% in 1991 and that it has since declined to 0.4% by 2009.