Longitudinal research by Harvard scientists over the course of 75 years may have found the number 1 secret to living a fulfilling life. Two studies were conducted to track the physical and emotional well-being of two populations.

The first was named the Grant Study and followed 456 poor men who grew up in Boston from 1939 to 2014. Glueck Study, the second, followed 268 male graduates from Harvard's classes of 1939 to 1944.

The entire study was named the Study of Adult Development. In its official website, the study intends to identify the psychosocial predictors of healthy aging and how biological processes from earlier in life predict well-being and health later on.

The length of the research period has led to the need for multiple generations of researchers. They have been tasked to analyze blood samples, conduct brain scans and check self-reported surveys.

Robert Waldinger, director of the Harvard Study of Adult Development, confirmed the result of the study. He concluded that good relationships keep people happier and healthier, Inc. reported.

Apparently, the biggest predictor of one's happiness and fulfillment in life is actually love. The study proves that people should have someone to rely on since this helps their nervous system relax and keeps the brain healthier for longer. It also reduces emotional and physical pain.

The study has also shown evidence that people who feel lonely are more likely to experience physical health decline earlier. They are also more likely to die younger.

Waldinger noted that it's not only about the number of friends that people have or whether or not they are in a committed relationship. The most important factor was the quality of those close relationships.

This means that the number of friends that a person has does not matter. What matters most is the depth of an individual's personal relationships and how safe you feel sharing with another person without fearing judgment.