Individual differences in willingness to engage in uncommitted sexual relations were investigated in 6 studies. In Study 1, a 5-item Sociosexual Orientation Inventory (SOI) was developed. Studies 2, 3, and 4 provided convergent validity evidence for the SOI, revealing that persons who have an unrestricted sociosexual orientation tend to (a) engage in sex at an earlier point in their relationships, (b) engage in sex with more than 1 partner at a time, and (c) be involved in relationships characterized by less investment, commitment, love, and dependency. Study 5 provided discriminant validity for the SOI, revealing that it does not covary appreciably with a good marker of sex drive. Study 6 demonstrated that the SOI correlates negligibly with measures of sexual satisfaction, anxiety, and guilt. The possible stability of, origins of, and motivational bases underlying individual differences in sociosexuality are discussed.