One of the most thorough research projects on relationships is called the Alameda County Study. Headed by a Harvard social scientist George Kaplan PhD, it tracked the lives of 7,000 people over nine years. Researchers found that the most isolated people were three times more likely to die than those with strong relational connections.
People who had bad health habits (such as smoking, poor eating habits, obesity, bad sleep habits, no exercise or excessive alcohol use) but strong social ties lived significantly longer than people who had great health habits but were isolated. In other words, it is better to eat Twinkies with good friends than to eat broccoli alone. Harvard researcher Robert Putnam notes that if you belong to no groups but decide to join one, “you cut your risk of dying over the next year in half.”
For another study, as reported in the Journal of the American Medical Association, 276 volunteers were infected with a virus that produces the common cold. The study found that people with strong emotional connections did four times better fighting off illness than those who were more isolated. These people were less susceptible to colds, had less virus, and produced significantly less mucous than relationally isolated subjects. (I’m not making this up. They produced less mucous. This means it is literally true: Unfriendly people are snottier than friendly people.)