What if we could study people from the time that they were teenagers all the way into old age to see what really matters to a person’s health and happiness? For 85 years (and counting), the Harvard Study of Adult Development has tracked about 2,000 men and women for three generations, asking thousands of questions and taking hundreds of measurements to find out what really keeps people healthy and happy.

Through all the years of studying these lives, one crucial factor stands out for the consistency and power of its ties to physical health, mental health, and longevity. It isn’t career achievement, or exercise, or a healthy diet. These things matter, but one thing continuously demonstrates its broad and enduring importance: good relationships

In fact, close personal connections are significant enough that if we had to take all 85 years of the Harvard Study and boil it down to a single principle for living, one life investment that is supported by similar findings across a variety of other studies, it would be this: Good relationships keep us healthier and happier. Period. If you want to make one decision to ensure your own health and happiness, it should be to cultivate warm relationships of all kinds.