For years, we’ve heard it taught that “your income will tend to equal the average of the income of your five closest friends.” Point being, if you want to raise your income level, hang out with people who are making some good money. I’ve also heard (and taught) the various corrolaries: Your mood will tend to reflect … Your level of happiness will tend to reflect … Your overall success will tend to reflect … all pointing to the same conclusion: choose your friends with care.

We wrote about this in The Slight Edge: Jeff calls it “The Law of Association”:

“Your level of health will tend to be about the average level of health of your five best friends. Your personal development will be at about the average level of personal development of your five best friends. Your relationships, financial health, attitudes, level of success in your career, and everything else about your life will tend to be very close to the average level of each of these conditions in your five closest friends and associates.” (The Slight Edge, from Chapter 9, “Mastering Yourself”)

Experience bears this out—but I’d never seen any actual scientific proof that this was so. At least, until this morning: the Law of Association now has an interesting tidbit of confirmation—from the New England Journal of Medicine, no less. As reported in The New York Times, a retrospective analysis of data from the famous 32-year Framingham Heart Study found that when “Obesity can spread from person to person, much like a virus.” When a subject’s friend became obese, even if living at a distance of hundreds of miles, the subject had a 57 percent greater chance of becoming obese him- or herself. And between “close mutual friends” the odds of the influence increased to 171 percent.

But not neighbors. And not even family members (at least, far less so). the influence, the study found, is passed specifically through friends. And check this out: “The same effect seemed to occur for weight loss, the investigators say.”

As the old knight said in Indiana Jones III, “Choose wisely.”