In 2015, two geographers noticed solar panels popping up on houses in their small US state of Connecticut. Curious, they set out to see if they could figure out what predicted who had them. Would they be in richer homes? Or in areas with higher population density?

Early adopters of solar panels tend to be people who are interested in innovative technology, who find an installer they trust, and who think having solar panels will benefit them.

But once an early adopter made their choice, the geographers found, a cluster would spring up around them. Having solar panels on a house near you, where you could see them and talk to a real live person who had them, it turned out, was the biggest predictor of whether you’d get them yourself. 

Soon the Connecticut study was being replicated – in Sweden, in China, and in Germany, where they actually put a number on it. Rooftop solar installations were most influential, they found, on neighbors who lived within one kilometer (source: TED ideas).

The truth, of course, is this applies well beyond installing solar panels. Solar panels are just physical manifestations of the proximity principle.

People who prioritize their health are more likely to have friends who prioritize their health. And so on.

We become like the people we choose to be around.