Why do people contribute to the well-being of others? What promotes or hinders their contribution? Framed by expectancy-value theory and the motivational theory of life span development, we use data from the Midlife in the United States National longitudinal study (MIDUS I, II, and III) to examine how individuals' perceived contributions to the well-being of others develop across adulthood, in the related but distinct forms of overall prosociality (more other-focused) and generativity (more self-focused). Our findings show that prosociality and generativity display similar, yet distinct trajectories, peaking in midlife a decade apart from each other, when expectancy and value for prosocial behavior are highest. Moreover, expectancy as reflected in perceived control and control strivings, and value as indicated by agreeableness, predict individuals' prosociality and generativity. Trajectories of prosocial contributions further differ according to individual differences in perceived control, control striving, education, income, and number of children, whereas trajectories of generativity only differ across levels of perceived control and income. By applying motivational and life span developmental perspectives, our study offers insight into how prosociality and generativity develop throughout adulthood. (PsycInfo Database Record (c) 2021 APA, all rights reserved).