Many of the studies also included income as an explanatory variable of air travel demand. This will isolate the effects of a shift along the demand curve (caused by a change in air travel price) from the effect of a shift of the whole demand curve (caused by a change in incomes or GDP). The studies including the income term all produced positive income elasticities, as would be expected (air travel increases as incomes increase). Virtually all of these studies estimated income elasticities above one, generally between +1 and +2. This indicates air travel increases at a higher rate than income growth. This has important implications for policies seeking to manage air travel demand by raising the price of travel