Despite consensus among medical authorities about the desirability of breastfeeding, causal evidence about its effects is scant. Using complementary empirical approaches and data from the National Longitudinal Survey of Youth spanning five decades, I investigate a comprehensive set of outcomes with greater breadth and continuity than previous work. On average, breastfeeding is associated with modest and persistent cognitive advantages from childhood through young adulthood—even after controlling for an extensive set of confounding forces. Accounting for breastfeeding duration strengthens these relationships and uncovers favorable labor market and fertility linkages as well. But there is no evidence for enduring health benefits. At the same time, a novel extended family fixed effects analysis comparing differentially breastfed siblings and cousins finds little association between breastfeeding and any outcome. These divergent results may reflect omitted variable bias in conventional estimates, but they may also be the consequence of considerable negative selection among the inconsistently breastfeeding families contributing to fixed effects identification.