PIP: A woman's contraceptive vigilance and her actual use of birth control on any given occasion frequently depend upon where the internal balance lies between her positive and negative feelings toward getting pregnant and between her positive and negative feelings about her current contraceptive method. This paper analyzes data from an empirical study of the psychological and behavioral aspects of sexuality, childbearing and contraception which was conducted, using a 3-year prospective design, to gather data from about 1000 women aged 18-27 living in the San Francisco Bay Area in California, US. Data were collected through interviews and self-report inventories. The study, known as the Psychology of Reproduction (POR), was divided into 3 subsamples: 325 never-married women; 322 recently married women with no children; and 320 married women who had just borne their 1st child. Each subject was interviewed 4 times: an initial interview, followed by 3 follow-ups conducted at 1-year intervals. The results indicate that among married women, expectations about future childbearing are a strong indication of underlying motivation. In addition to the conscious motivations underlying childbearing expectations, there are 3 types of subconscious motivation. Among never-married women, expectations about future childbearing are relatively less likely to affect contraceptive practice because social norms tend to support childbearing only within marriage. Belief in modern roles for women tends to be associated with negative maternal motivation, while identification with traditional roles is associated with positive maternal motivation. Specific contraceptive attitudes may have only an indirect effect on contraceptive nonuse, through their influence on a woman's choice of a method. Among married women, use of the pill or the IUD is associated with lower levels of nonuse, while among both married and never married women, use of a barrier or ad hoc method is associated with increased nonuse. How the husband feels about the method the couple is using and the presence of conflict in the marriage are 2 psychological mechanisms suggested by the findings for married women. Behavioral stability and educational level indicate that a woman's ability to integrate knowledge is important for understanding nonuse behavior.