Population differences exist in personality and sexual behavior such that, in terms of restraint, Orientals > whites > blacks. This ordering is predicted from an evolutionary theory of r K reproductive strategies in which a tradeoff occurs between gamete production and parental care. Literature is reviewed on differences between the three groups in rate of ovulation, intercourse frequencies, sexual attitudes, developmental precocity, size of genitalia, secondary sexual characteristics, and biologic control of behavior that accord with this formulation. Novel analyses of data from the Institute for Sex Research are also carried out, indicating that American blacks, compared to American whites, are more precocious and less restrained. Their parents were younger when they were born, had more children, and had an earlier mortality. Blacks left home earlier, experienced a variety of premarital, marital, and extramarital sexual events earlier, had a greater number of premarital and extramarital partners, had a greater frequency of marital intercourse, used fewer contraceptives, and had a greater incidence of pregnancy, at a faster rate. The men had larger penises, at a different angle of erection, and maintained intromission longer, while the women had shorter menstrual cycles, more periodicity of sexual response, and a greater number of orgasms per act of coitus. Whites varied their sexual activity more, both with spouses and with prostitutes, although blacks consorted with prostitutes more frequently. Finally, blacks had a shorter duration of marriage and more permissive sexual attitudes.