I left for vacation on July 24. Just two days later, a story broke that made me wish I could ditch the family and return to blogging, if only for a few hours. When I checked my email, I saw that at least a half-dozen readers had sent me links to the story. (Thanks to all, by the way.)

You probably know the summary already. A study published in the New England Journal of Medicine found that obesity is spread through social networks -- that is, friends let friends become fat. Here's a snippet from Gina Kolata's report in the New York Times last Thursday:

I'm hardly an expert on research techniques, but the design of the study strikes me as ingenious. The authors went back through decades' worth of handwritten records in the Framingham Heart Study. Their goal was to find "alters," or people who were listed as close friends and relatives of the participants in the study itself. (I can't say this for certain, but I believe the friends' names were originally recorded to help the researchers find participants who have moved or dropped out of the study or in some other way become difficult to track.)

They started with about 5,000 Framingham participants and another 7,000 alters. They then built out social networks that included 38,611 individuals, or about 7.5 per participant.

Some bullet points:

• Men make men fat: If a close male friend becomes obese, your odds of becoming obese increase 100 percent.
• Women, conversely, are nearly immune to their best friends' obesity; their transmission rate is only 38 percent.
• Opposite-sex friends don't affect each other's weight.
• However, sisters have a much bigger influence on each other's weight than brothers: A woman is 67 percent more likely to become obese if a sister she's close to reaches that standard. For brothers, it's 44 percent. The effect only occurs if the siblings regard themselves as close; if you hate your family, they probably don't affect your obesity risk, aside from whatever propensity is built into your genetic hardware.
• You'd think that spouses would have a dramatic influence on each other's weight. But your chance of becoming obese if your wife does is just 37 percent.
• Neighbors don't have any measurable effect on obesity. The researchers didn't say this, but I'd guess it really depends on how much you like your neighbors. That's why Ned Flanders remains relatively slim despite the fact he lives next door to an obese person.

Interestingly, the call to maintain close friendships despite health risks would be ridiculed if we were talking about anything besides obesity. If you knew your 15-year-old daughter was hanging out with a group of girls who were sexually active, including several who'd become pregnant, wouldn't you be alarmed? Wouldn't you do anything in your power to get your daughter away from girls who considered unplanned, underage pregnancy the normal course of events?

What if your son was running with a group that included obvious drug users, or petty criminals, or guys who liked to get drunk and smash things up on a Saturday night?

You'd be considered negligent if you didn't intervene. You might switch schools, if that was affordable or feasible. You might even move to a new area. At a bare minimum, you'd push your kids into different activities, with different peers.

Please don't misunderstand: I'm not equating obesity with criminal or antisocial activity. But when we talk about obesity, we are talking about the potential for lifelong health problems.

Maybe it's better to look at this in terms of ambition and aspiration. If your best friends from high school, say, are all career-oriented, what are the odds that you'd be a slacker in a dead-end job? Pretty low, I'd think. You'd either find a different job or different friends.

Same with personal habits. If you're the only smoker in your social circle, wouldn't you feel some motivation to quit? But if, on the other hand, all your friends were smokers and you decided to quit, you know you'd have to make a hard decision regarding those friends of yours. Your only real chance to quit the habit is to quit (or at least modify) the relationships that encourage you to keep smoking.

This is all intuitive and validated by every single person's social history -- yours, mine, everyone's. I don't smoke or drink to excess, so I don't have any friends who do those things. The people I'm close to are all over the place in terms of weight, but everyone I can think of is health-conscious.

I assume social scientists would agree that interpersonal networks are key to normalizing behavior, good or bad. You can't choose your family, but you can choose your friends, as the saying goes. It's not a stretch to say you can choose your lifestyle and personal habits, as well.

That's not profound, of course, but it's the best I can do on my first day back.

Post-vacation blog meat

• caffeine + exercise = lower risk of skin cancer. As a fan of both, I was way ahead of the curve on this one.
• Just what the world needs: schizophrenic mice.
• Don't bogart that joint -- a new study finds that the lung damage from a J is equal to that caused by chain-smoking five cigarettes. Obviously, the solution is to share the buzz.