How can we tell if happiness leads to better health? Any study of this question is plagued by the chicken and egg problem. Which came first, happiness or good health? There are many studies showing cross-sectional correlations between happiness and health. In other words, at any given point people who are happier are also healthier. While these studies show a clear relationship between positive mood and physical health, we cannot know which came first. Unlike cross-sectional studies, however, longitudinal studies can show whether high levels of positive mood actually precede healthy outcomes. For example, in a recent study of five thousand people, high levels of positive mood predicted to fewer hospitalizations five years later and a lower incidence of stroke six years later.