I am getting fat. I was thin when I lived in LA and NY because people in those cities are thin. People in rural America are fat. Really fat. And I’m getting fat living with them.

You become who you spend time with. Sometimes I coach college kids who want to go to law school to work for the ACLU, or defend poor people from ills of society or something like that. I say, “How will you pay off your law school loan?”

They always say they will work in a big law firm, pay off the debt, and then become a do-gooder. But it never happens like that because even if you are not a money-grubbing power-hungry lawyer type, you are surrounded by those people in big law firms because why else would they spend thirty years doing corporate law? And even do-gooders become who they spend time with.

We know we become like the people we spend time with from the Framingham Heart Study.  It’s a famous, long-term study about heart disease where we accidentally learned tons of information about how people live. The study participants are nurses and they’ve been meticulous about reporting reliably year after year for decades.

We know from the study, for example, if your friends drink, you will probably drink. (Note: I started drinking when I moved to Wisconsin, which has a huge alcohol culture.) If your friends are fat you will probably be fat. Not just nurses, but everyone. Even lawyers!

I notice that I don’t choose who I spend time with randomly—I tend to choose intellectual, low-maintenance friends. (Low maintenance as in I can refuse to answer my phone for a week and they don’t take it personally.) If I were surrounded by very emotional people I don’t think I’d be able to make a friend.

So I wonder about putting kids into a perfectly balanced classroom where there are boys and girls and intellectuals and sports stars, and kids have to search to find someone like themselves. I’m not sure that is really how the world works.

First, I think we tend to surround ourselves by people like us. Just like we hire people who are like us. But also, if we are in a place where people are not like us, we change to become more like them.

(Which brings me back to getting fat, of course.)

The same is true of kids. If they hang around kids who are fat, the kids will become fat.

A sociological study of kids in school shows that kids make friends not based on who they are or what they are interested in, but rather, who is in close proximity to them. Who is in math class, for example.

If you put kids in school to get exposed to kids who are different from them, all the kids will melt toward the mean, no matter how smart or talented or fat they are.

But if you let kids choose their own context, then they do, in fact, choose their own friends. Just like it works in the adult world.

It turns out that if you let kids eat whatever they want, they are likely to choose what’s good for them. And I have a feeling that’s true for friends as well.