During the development process, the level of education of the population tends to increase steadily. In Europe, a significant proportion of the population knew to read and write at the beginning of the 19th century. Then, schooling became compulsory and universal (at the end of the 19th century in France). The level of education continued to increase during the whole 20th century because of the increasing length in duration of schooling. At the beginning of the 21st century, the average number of years of schooling reached high values in developed countries, ranging from 9 to 12 depending on the country.