THE hardest vices to shake are those that come cheap. For all the efforts to kick the world’s collective addiction to inexpensive fossil fuels, energy from renewable sources still makes up less than a fifth of global energy production. Yet the world is gradually moving towards a greener future. What will this future look like? Nuclear power, expensive and potentially hazardous, has fallen out of vogue. Erstwhile nuclear converts such as Japan and Germany have been phasing out nuclear plants. The road to low carbon emissions will probably be lined with wind turbines and photovoltaic panels.

Solar production, in particular, is exploding in America. Capacity increased tenfold from 2009 to 2015, and installations have grown by 65% per year since 2000, a stunning feat. Yet one obstacle remains: although manufacturing costs have fallen sharply, installing solar panels is still expensive.

One effective remedy for high installation prices may be peer pressure. Tendril, an energy-intelligence firm, crunched the numbers on solar-panel installations in San Jose, California since 2001. The company developed a machine-learning model to sort out which factors were most salient in predicting an installation, using an impressive data set that included mutual-fund investment, interest in the outdoors and “high-life behaviour”.

Among all these, the most likely predictor of having a solar panel was having a neighbour who had installed one. Income came in second, its predictive power only half as strong. Keeping up with the Joneses used to mean matching the (often environmentally destructive) conspicuous consumption of one’s neighbours. Following the crowd might help the planet as well—at least in the green-leaning cities of California.