Wealthier countries exhibit a negative relationship between income and weight, or a greater demand for thinness. When food prices are low, people spend a smaller share of their income on food. The tendency is to consume more calories, but only up to a certain income level at which concern for being above ideal weight will limit calorie consumption. Poorer countries, by contrast, exhibit a positive relationship, or greater demand for obesity. When food prices are high, people spend a larger share of income on food.
Obesity thus becomes a physical symbol of wealth.
While technological advancement does spur demand for increased calorie intake, the effect is self-limiting. Preferences for weight change with stage of development and corresponding income level. As increase in calorie consumption does not provide a full explanation for the rise in obesity, Philipson and Posner also discuss a second effect of technological change on caloric expenditure. Technological change has lowered the quantity of calories expended at the work place. People must therefore pay to undertake physical activity by forgoing leisure.