It has been argued that the consumer has benefited from agricultural technology in the United States. Consumers are spending, on the average, a decreasing proportion of their income on food. But the demand for food, especially in prosperous countries, is virtually insensitive to income, so that as real incomes rise, the amount spent of food becomes a smaller proportion of expenditure. Therefore, in order to judge whether consumers have really benefited from agricultural technology, it is necessary to know whether ______.