The study began in 1938, at the height of the economic crisis that hit the world and became known as the "Great Depression", and proved a strong link between friendships and happiness, and the consequent improvement in health.
The study included questions about public life, including health, career and marriage.
According to Sky News, the study found that good friends played a role in protecting people from mental and physical deterioration, compared to the social class and intelligence level.
"The sudden discovery is that our relationships and our happiness with these relationships have a strong impact on our health as well," said Robert Dingger, psychiatrist at Massachusetts Hospital. "Taking care of the body is important, but caring for relationships is also a kind of self-interest."