The growing popularity of the automobile, and corresponding changes in dating practices, caused premarital sex to become more prevalent. Alfred Kinsey found that American women who became sexually mature during the 1920s were much less likely to be virgins at marriage than those who became mature before World War I. A majority of women during the 1920s under the age of 30 were nonetheless virgins at marriage, however, and half of those who were not only had sex with their fiancés. A 1938 survey of American college students found that 52% of men and 24% of women had had sex. 37% of women were virgins but believed sex outside marriage was acceptable. Prior to the middle of the 20th century, sexuality was generally constrained. Sexual interactions between people without plans to marry was considered unacceptable, with betrothal slightly lessening the stigma. However, premarital sex was still frowned upon.