Yet this argument has a flip side: If consumers of these products die earlier, they will also collect less in pension payments, including Social Security. Economists have run the numbers for smoking and often find that these savings may more than offset the budgetary costs. In other words, smokers have little net financial impact on the rest of us.

It may seem grisly to consider the budgetary savings of an early death as a “benefit” to society. But when analyzing policy, economists are nothing if not cold-blooded. If one uses budgetary costs to justify taxing particular consumption goods, the accounting needs to be honest and complete.

There is, however, an altogether different argument for these taxes: that when someone consumes such goods, he does impose a negative externality  on the future version of himself. In other words, the person today enjoys the consumption, but the person tomorrow and every day after pays the price of increased risk of illness.

This raises an intriguing question: To what extent should we view the future versions of ourselves as different people from ourselves today?

To be sure, most parents have no trouble restricting a child’s decisions on the grounds that doing so is in the young person’s best interest. Few teenagers are farsighted enough to fully incorporate the interests of their future selves when making decisions. As parents, we hope that someday our grown-up children will be grateful for our current restrictions on their behavior.

But people do not suddenly mature at the age of 18, when society deems us “adults.” There is always an adolescent lurking inside us, feeling the pull of instant gratification and too easily ignoring the long-run effects of our decisions. Taxes on items with short-run benefits and long-run costs tell our current selves to take into account the welfare of our future selves.