Economists studying cities routinely find that after controlling for other variables, workers in denser places earn higher wages and are more productive. Some studies suggest that doubling density raises productivity by around 6 percent while others peg the impact at up to 28 percent. Some economists have concluded that more than half the variation in output per worker across the United States can be explained by density alone; density explains more of the productivity gap across states than education levels or industry concentrations or tax policies.
Excerpted from: One Path to Better Jobs: More Density in Cities
h/t: Strong Towns Blog