Until the 19th century, single-sex education was the norm in the US, although this varied by region. In New England there was more mixed-sex education than in the South, and girls in New England had more access to education in general. With the generalization of elementary education in the 19th century, mixed-sex education started to spread rapidly. According to Cornelius Riordan, "By the end of the nineteenth century, coeducation was all but universal in American elementary and secondary public schools (see Kolesnick, 1969; Bureau of Education, 1883; Butler, 1910; Riordan, 1990). However, higher education was usually single-sex, and men's colleges and women's colleges were common well into the 20th century. A form of education strongly associated with sex-segregation is that of Catholic schools, although many Catholic schools today are coeducational. The idea of educating students differently by sex, formally or informally, was common until the 1970s.