This study takes comparative evolutionary approaches to relative contribution of various factors to women’s reproductive behaviour in low-fertility societies like China. A series of theoretical hypotheses about second-birth intentions and behaviour—a key to understanding low-fertility behaviour—are raised and then tested by dominance analysis of longitudinal data from a sample of one-child mothers in China. It is found that behavioural ecology approach has the largest explanatory power. All members within nuclear family were complete stakeholders of reproduction: Husband’s fertility attitude, i.e. injunctive norms of him as perceived by wife, made the largest contribution to fertility intentions, followed by wife’s own attitudes, which were further followed by firstborn’s. Among incomplete stakeholders, injunctive norms of peer relatives and friends contributed less to fertility intentions than parents’, but the opposite held for descriptive norms, i.e. actual number of children. Regarding the actual behaviour followed over 2.5 years, fertility intentions were the dominating predictor of it; husband’s, firstborn’s and wife’s fertility attitudes were equally important predictors; neither injunctive nor descriptive norms of other social-network members were significant contributing factors. Perceived challenge in investing in children was an important factor for both fertility intentions and behaviour and other constraints only became important at the latter stage. The study consolidates the theoretical foundation of collective decision-making in family reproduction, helps to clarify kin influence on women’s fertility, suggests cultural evolution of fertility by horizontal transmission of new pronatalist norms in current China, and further supports comparative evolutionary approaches to reproductive behaviour and fertility policies in modern low-fertility societies.