Teen sex was rarely talked about in the 19th century, but that changed in the 20th with the coming of new sexual mores and the growth of public high schools, which brought girls and boys together in an institutional setting that fostered greater contact and intimacy than ever before. In 1900 probably less than 10 percent of Americans ages 15 to 18 were in public high schools, but by 1940 the proportion had grown to about two thirds.

Not surprisingly, in the 21st-century U.S. many teenagers admit to loss of virginity. This behavior becomes problematic when young people fail to use contraception. A quarter of 15- to 16-year-old girls, for example, had unprotected sex during their first intercourse. (This statistic, however, marks an improvement over the pre-1980s level of 57 percent.) Only half the parents of girls younger than 18 counsel their daughters on how to say no to boys, and only about half talk to them about birth control. Teenage girls from intact homes are less likely to have had sexual intercourse.