This was not only due to the difficulty of obtaining information on sexual behavior, but also to the assumption that intercourse takes place primarily within marriage. In the past, fertility was studied primarily within marriage. Since under all but exceptional circumstances, conception does not occur without it, sexual intercourse is the first of these proximate factors to be examined.

In their theoretical analysis of fertility and its determinants, Davis and Blake (1956) argued that socioeconomic and other factors affect fertility only through its proximate determinants, that is, through exposure to sexual intercourse, exposure to conception, given intercourse, and gestation and successful parturition, given conception.

The initiation of sexual intercourse is an important topic in the study and prediction of fertility.