I find this perfectly credible because almost exactly the same conclusions were stated by Putnam in the classic _Bowling Alone_ [1]. A couple of pull-quotes from that,
> Dozens of painstaking studies... have established beyond reasonable doubt that ... [t]he more integrated we are with our community, the less likely we are to experience colds, heart attacks, strokes, cancer, depression and premature death of all sorts...
> ... the positive contributions to health made by social integration and social support rival in strength the detrimental contributions of ... risk factors like ... smoking, obesity, elevated blood pressure, and physical inactivity.
> ...as a rough rule of thumb, if you belong to no groups but decide to join one, you cut your risk of dying over the next year in half.
Putnam was surveying a large number of studies, not just the Harvard one.
> Dozens of painstaking studies... have established beyond reasonable doubt that ... [t]he more integrated we are with our community, the less likely we are to experience colds, heart attacks, strokes, cancer, depression and premature death of all sorts...
> ... the positive contributions to health made by social integration and social support rival in strength the detrimental contributions of ... risk factors like ... smoking, obesity, elevated blood pressure, and physical inactivity.
> ...as a rough rule of thumb, if you belong to no groups but decide to join one, you cut your risk of dying over the next year in half.
Putnam was surveying a large number of studies, not just the Harvard one.
[1] Putnam, Robert D, Bowling Alone: The Collapse and Revival of American Community; https://www.amazon.com/Bowling-Alone-Collapse-American-Commu...