Seventy five years in the making, the longitudinal study of 268 male Harvard undergraduates, called the Grant Study of Adult Development can be found in George Vaillant’s Triumphs of Experience. One take away: The correlation between warmth of your relationships and your health and happiness when older. While the study concludes that one can recover from a lousy childhood, memories of a happy childhood are a lifelong source of strength. In the author’s own words: “Happiness is love. Full stop.”