In 1937, a researcher at Harvard University began a study (originally named The Harvard Study of Adult Development) on what factors contribute to human well-being and happiness. The research team selected 268 male Harvard students who seemed healthy and well adjusted to be part of what is called a longitudinal study, which means that the researchers would study the lives of these men not just at one point in time, but rather over a period of time. In this case, the period of time in what is now called the Grant study has been extraordinary: 72 years. With 72 years of perspective, the Grant study gives a comprehensive viewpoint on what has affected the level of health and happiness of men over a lifetime.

The Grant study has tracked an array of factors, including standard measurable items like physical exercise, cholesterol levels, marital status, the use of alcohol, smoking, education levels, and weight, but also more subjective psychological factors such as how a person employs defense mechanisms to deal with the challenges of life.

Over the period of 72 years, several men have directed the research. For the last 42 years, the director has been psychiatrist George Vaillant. In 2008 someone asked Dr. Vaillant what he had learned about human health and happiness from his years of poring over the data on these 268 men. You would expect a complex answer from a Harvard social scientist, but his secret to happiness was breathtakingly simple: "The only thing that really matters in life are your relationships to other people."