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An analysis of the hub-and-spoke network examined the route networks of the 13 major air passenger carriers in the US in 1989. Based on route-specific data, these airlines were classified according to geographic scope, scale of operations and type of route structure. Two had regional coverage and the rest had a more national scope. Among the national carriers, variation was found in the number of routes they serve. Among all the carriers, variation was observed in the degree of centralization. This analysis also ascertained the location of airline hubs and investigated the extent of competition on particular routes, for individual airlines and at individual airports. Findings revealed 46 airport-airline combinations that can be considered as hubs. These combinations involved only 37 airports. Hub airports were found to aggregate within 500 miles of Cincinnati and were located in densely populated areas.

Barro-style models of endogenous growth imply that economic growth will initially rise with an increase in taxes directed toward economically "productive" expenditures (e.g., education, highways, public safety), but will subsequently decline—consistent with a "growth hill"—as the rising tax share depresses the net return to private capital. Previous tests focus on whether the linear incremental effect of taxes is positive, negative, or zero, with substantial evidence for all three conclusions. This study incorporates potentially non-monotonic effects for fiscal policy. Based on estimates for U.S. states, the incremental effect of taxes directed toward publicly provided productive inputs is initially positive, but eventually turns negative, consistent with a growth hill.