There are more reasons for homeowners to install solar than ever before, and a new study suggests keeping up with the Joneses may be one of them.

SolarCity recently released data regarding a 'contagion' effect, where one homeowners installs solar panels and numerous others in the neighborhood follow suit. Roughly one-third of customers were referred by a friend or neighbor, says the largest solar installer in the U.S. SolarCity provided a few graphics that have shown the growth of solar panel installation in states like Hawaii, California, Oregon, Arizona, Texas, New Jersey, and Massachusetts.

Similarly, a paper in the Journal of Economic Geography analyzed the contagion effect in Connecticut, where researchers found a centrifugal pattern of solar panel installation.

The researchers, Marcello Graziano of the University of Connecticut and Kenneth Gillingham of Yale, tried to figure out why solar power would expand in this way. Maybe solar power was just concentrating in large population centers? Nope: In fact, solar power was growing more rapidly in small and medium population areas. Nor was solar power simply proliferating in the rich parts of the state; households of all income levels were adopting solar power.

Solar contagion makes some intuitive sense: Most people still don't know very much about residential solar power, how the subsidies work, or whether it makes financial sense. The technology is still new. But if they see that their neighbor is putting up panels, suddenly the possibility becomes more concrete.

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