Author Clive Thompson seems rather naive at times:
"But how, exactly, could obesity or happiness spread through so many links? Between one immediate peer and another, some contagious behaviors — like smoking — seem pretty commonsensical. If lots of people around you are smoking, there’s going to be peer pressure for you to start, whereas if nobody’s smoking, you’ll be more likely to stop. But the simple peer-pressure explanation doesn’t work as well with happiness or obesity: we don’t often urge people around us to eat more or implore them to be happier."
A lot of the participants in the Framingham study are Italian-Americans, such as the Belliolis profiled in the article's opening paragraphs, and I can assure Clive Thompson that Italian-Americans do often urge people around them to eat more and to be happy. Indeed, eating and happiness is often linked in the Italian-American mind. (Whazzamatta, you don't like your grandmother's canoli? You don't love your own grandma anymore? C'mon, enjoy, enjoy. Be happy!). Similarly, Irish-Americans do often urge the people around them to have a wee nip or three.
This is one reason why parents spend so much money and effort on trying to get their kids into higher social classes with, among much else, healthier habits. For example, when I arrived at Rice University at age 17 way back in 1976, there were only 3 smokers out of the 250 guys in my dorm. Rice students weren't particularly upscale (they tended to be the children of engineers), but they tended to have Mr. Spock-like views on the obvious illogicality of smoking. Not surprisingly, I was never subjected to any peer pressure to start smoking.
Only on the 10th and last page of the article does the author finally get around to considering selection effects on the results, and he could go much farther. Consider weight -- so many of the rituals of friendship revolve around exercising and/or eating, drinking, or taking drugs. Consider two young women who are best friends because they go out frequently to Manhattan dance clubs together and try to lure men into buying them cocaine. If they weren't both slender, they wouldn't have taken up this hobby, and, in turn, this hobby keeps them skinny (until one, or both, goes into rehab).
Consider two male friends who get together most nights to down a six-pack of beer each and watch ESPN. If one of the pair got into triathaloning, they'd probably drift apart.
To my mind, perhaps more interesting than friendship ties are the social influences of kinship ties, which bring together people of different ages, sexes, and personalities. (You can choose your friends, but you can't choose your relatives.) In-law ties are particularly complex and interesting, but understudied. One incomplete but illuminating definition of "class" is "the kind of people your relatives tend to marry."