What makes you fat? Eating cheesy-poofs while watching Sex in the City reruns? Wolfing down a Wendy’s “Baconator,” comprising a double cheeseburger with six strips of bacon that could feed everyone in Darfur for a week? How about when you get the urge to exercise you lie down until it goes away, as one CEO famously put it?

Yes, to all of the above. But these are all specific contributors to obesity driven by larger forces that are making us, well, larger. One of the most important of these, as a new study in the New England Journal of Medicine shows, is that having fat social contacts makes you fatter. Obesity is contagious.

It’s unfortunate that while the response to this should be “Well, duh!” it’s being treated as a revelation on par with the announcement that Pluto was demoted from planet status. Not to toot my own horn (or perhaps, just a bit), but all this was known a decade ago because I wrote about it in my 1997 book The Fat of the Land.

I’ve written that overweight and obesity isn’t just the individual problem of lots of Americans (two thirds, actually), but even aside from the direct costs imposed on all of us including higher Medicare and Medicaid expenditures and costlier private health insurance premiums, it’s both an individual and a national problem and should be treated as such.

The basic premise of obesity as contagion is simple: The more something becomes prevalent, the more it becomes acceptable and the more of it you get. In a vicious cycle, more divorces begat more divorces, more unwed pregnancies begat more unwed pregnancies, more tattoos and piercings begat more self-mutilation (er, “body art”) and so on. Obesity isn’t just a physiological problem of too many calories in and too few out; it’s a long-term social problem.

In the study, researchers at Harvard and the University of California at San Diego followed a large social network of about 12,000 people over 32 years. In other words, this is anything but a tiny sampling. The group included friends (whether they lived nearby or not), spouses, siblings, and neighbors. The fatter these were, the fatter the index person became.

And lest you jump at the “fat gene tomfoolery, it turned out that the greatest influences were friends and not family. The study found a person’s chances of becoming obese went up 57 percent if a friend became so, though only 40 percent if a sibling became so. It went up 37 percent if a spouse became obese and, in the closest friendships, the risk almost tripled.

The study did not look at the relationship between parental and child obesity, but others have. Indeed, the New England Journal of Medicine has reported that parental obesity dramatically increases the likelihood of child obesity. The explanation was that “children imitate their parents’ eating and exercise habits.” Yes, another “Well, duh!”

Mind you, nobody wants to be fat, notwithstanding the fat acceptance advocates who express “pride” in weighing 350 pounds or more and seem to think they deserve medals for downing Twinkies by the score. It’s just that people make an unconscious decision that they’d rather engage in (or when it comes to exercise, not engage in), activities that make them fat rather than conducting themselves in such a way as to stay or become thin.

“If you’re just a little bit heavy and everyone around you is quite heavier, you will feel good when you look in a mirror,” as David Katz physician and director of the Yale-Griffin Prevention Research Center, explained to the Associated Press.

So what to do with this not-so-new knowledge?

Quarantining two-thirds of the population to protect the other third is not the sort of thing compassionate conservatives advocate. It’s also rather impractical. But sometimes simply increasing awareness with public service advertisements can be a powerful tool. While I’ve long criticized the “science” of so-called “second hand smoke,” I’ll readily grant the PR campaign was succeeding in getting active smokers to quit long before the first indoor smoking ban passed.

Daddy might think twice about consuming a side of beef between two buns if he realizes his eating and consequential belt-covering paunch may condemn his children to the rapidly growing ranks of type 2 diabetes sufferers.

And we need to patiently explain to those libertarians who cry “Nanny state!” and “Food Police!” whenever the CDC or Surgeon General mentions the obesity epidemic that calling attention to something and effecting a bacon ban are two different things. It has always been the job of the public health community to, well, protect the public health. That the greatest threats have gone from being infectious diseases to lifestyle diseases doesn’t change that.

And finally there’s the food and beverage lobby, led by the Center for Consumer Freedom, which claims there’s no obesity epidemic, that if there were it would be caused by lack of exercise not by caloric intake, and finally it would be marvelous if there were because being overweight is actually good for you. For them, we must invoke the immortal words of Ring Lardner. “‘Shut up’ he explained.”