Workers in cities earn 33 percent more than their non-urban counterparts. A large amount of evidence suggests that this premium is not just the result of higher ability workers living in cities, which means that cities make workers more productive.
Evidence on migrants and the cross-effect between urban status and experience implies that a significant fraction of the urban wage premium accrues to workers over time and stays with them when they leave cities. Therefore, a portion of the urban wage premium is a wage growth, not a wage level, effect.
This evidence suggests that cities speed the accumulation of human capital.
Glaeser, Edward and David C. Maré. 2001. "Cities and Skills," Journal of Labor Economics, 19:2, pp. 316-42.