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Academic studies have shown that the major factor of obesity among children and adults is excess availibility and low prices of soda drinks and fast food. In addition, large exposure of vending machines in schools also contributes to obesity through the consumption of candies and other high-calory products. For instance, an interesting study has examined the effect of vending machines on body weight. It showed that people with direct access to vending machines in schools and workplace gained cca. 3 kg more than those without access to vending machines. Therefore, is a tax on soda drinks and fast food a viable way to contain the growth of obesity?
Greg Mankiw has presented the economic point of view in NY Times (link). Generally speaking, the demand for soda drinks and fast food is price elastic, which means that a 1 percent decrease in the price of soda drinks would increase soda drink consumption by more than 1 percent. If government policymakers introduced a tax on soda drinks (fast food included), the burden of the tax would shift entirely on consumers who would pay the eventual cost of health care arising from obesity as a consequence of excessive consumption of soda drinks and fast food. That is why such a tax is feasible. There is no reason why the society should bear the burden of obesity through higher tax burden and rising cost of health care delivery. This is the most obvious evidence how society pays for private action that should be tackled by individuals themselves.