It seems to me that the 'force' we need to postulate account for the central
role of cities in economic life is of exactly the same character as the 'external
human capital' I have postulated as a force to account for certain features ofaggregative development. If so, then land rents should provide an indirect
measure of this force, in much the same way that schooling-induced earnings
differentials provide a measure of the productive effects of internal human
capital. It would require a much more detailed theory of the external effects of
human capital than anything I have provided to make use of the information
in urban land rents Gust as one needs a more detailed theory of human capital
than that in section 4 to utilize the information in earnings data), but the
general logic is the same in the two cases. What can people be paying
Manhattan or downtown Chcago rents for, if not for being near other people?