Under the Rana family, which ruled Nepal from 1846 to 1951, only the upper class had access to education. After the 1951 revolution, Nepal established an education system with free primary education for all children. Primary school begins at the age of 6 and lasts until age 10. Secondary education that follows lasts until the age of 15. Attendance of primary school was near universal for boys, but only 84 percent of primary school-aged girls were enrolled in 1996. Secondary school enrollment included only 37 percent (48.7 percent of the boys of that age group and 25 percent of the girls) in 1996. Formal schooling in Nepal is constrained by economic and cultural factors such as a bias against educating girls and a need for children to work at home or in the fields. In 2001 the literacy rate was estimated at 61 percent of the adult population, with a large gap between male and female literacy rates. Only 44 percent of the female population was literate in 2001 compared to 77 percent of the males. Urban areas have higher literacy rates than rural areas. In 1990 Nepal launched a 12-year literacy program targeting 8 million people between the ages of 6 and 45 years old. Tribhuvan University, founded in Kathmandu in 1959, is the only doctoral-granting institution of higher education in Nepal. Nepal also has a number of colleges, all of which are either affiliated with, or follow standards set by, Tribhuvan University.