"For the U.S. population, the sources of first ejaculation are, in order of frequency, masturbation (in about two-thirds of the males), nocturnal emissions (in an eighth of the cases), heterosexual coitus (in one boy out of eight), and homosexual contacts (in one boy out of twenty), with spontaneous ejaculation, petting to climax, and intercourse with other animals as less frequent stimuli for the initial experience (cf. Rodleder 1921). There are considerable differences in first sources in different educational levels. The highest incidence of masturbation as the first source of ejaculation occurs among the boys who will leave school between the ninth and twelfth grades, the highest incidence of nocturnal emissions as the first source occurs among the boys who will subsequently go to college, and the highest incidence of heterosexual intercourse as the first source occurs among the boys who never get beyond the eighth grade in school."