There is a large amount of cross‐sectional evidence for a midlife low in the life cycle of human happiness and well‐being (a ‘U shape’). Yet no genuinely longitudinal inquiry has uncovered evidence for a U‐shaped pattern. Thus, some researchers believe the U is a statistical artefact. We re‐examine this fundamental cross‐disciplinary question. We suggest a new test. Drawing on four data sets, and only within‐person changes in well‐being, we document powerful support for a U shape in longitudinal data (without the need for formal regression equations). The article’s methodological contribution is to use the first‐derivative properties of a well‐being equation.