But as this study — the Grant Study — progressed, the power of relationships became clear. The men who grew up in homes with warm parents were much more likely to become first lieutenants and majors in World War II. The men who grew up in cold, barren homes were much more likely to finish the war as privates.

Body type was useless as a predictor of how the men would fare in life. So was birth order or political affiliation. Even social class had a limited effect. But having a warm childhood was powerful. As George Vaillant, the study director, sums it up in “Triumphs of Experience,” his most recent summary of the research, “It was the capacity for intimate relationships that predicted flourishing in all aspects of these men’s lives.”

So it really is true: All you need is love.