In New York City, 21 is the new 18—and life for smokers is even more miserable.

A new bill approved yesterday by the city council makes 21 the legal age for buying tobacco. The minimum price of a pack of cigarettes or little cigars is now $10.50, the highest of any big city in the country. And it is now illegal to offer "discounts" on tobacco products.

Mayor Bloomberg, having banned the presence of cigarettes in practically every square inch of the city over which he has jurisdiction, notches another victory in his long war against smoking. But this particular battle is all about the kids. After all, nine in ten smokers started before they turned 18. Most teens know that smoking is bad for them, and only 5 percent expect to still be smoking within 5 years. But, five years later, 75 percent of them are still smoking.

So the question is, does this sort of thing--higher tobacco taxes and other interventions--do anything? Does it actually reduce teen smoking?

The research here is complex, but fascinating. The first data point you have to know is that cigarette smoking has declined tremendously in the last half-century--by almost half since the 1950s.

It's fallen just as dramatically for young people in just the last decade. A CDC study from last year found that from 2000 to 2011, the share of middle schoolers using tobacco by half, from 15 percent to 7 percent. Among high school students, cigarette smoking fell from from 28 percent to 16 percent.

The reason behind smoking's decline, to the extent that we can identify one, is probably a mix of culture and economics. The public is more aware today of the negative health effects of smoking than ever before. At the same time, state and local taxes have pushed the average cost of a pack of cigarettes from nearly $1 in 1970 to nearly $6 today. Today, adults born in low-cigarette-tax states are still much likelier to smoke as adults, according to economist Jonathan Gruber. Researchers who have exhaustively studied the impact of tobacco prices on consumption, have found that a 10 percent increase in cigarette prices decreases overall consumption by about 5 percent. 

But what about the kids?

Economists and psychologists have labored to understand exactly what makes young people start and stop smoking. Some studies have shown that young people, having little money of their own, are three times more sensitive to price changes than adults. That is, a 10 percent increase in cigarette prices would decrease their consumption by 15 percent. But another 15-year study from the Fred Hutchinson Cancer Research Center showed that there was no impact from the intervention programs in reducing youth tobacco usage.

In short, we don't know whether the new smoking age will change behavior among New York teenagers, since most smokers start smoking before they turn 18, anyway. But the body of evidence suggests that higher and higher prices for tobacco is an effective way to discourage smoking at a young age, which is when the majority of smokers (not expecting to be signing up for a long-term addiction) first light up. And, as Gruber has found, the lifetime savings of stopping a 16-year-old from buying that first pack can be immense. "The cost of smoking one pack of cigarettes, in terms of the value of life lost, is $35 per pack," he writes.

Sometimes, a little tax can go a long way.

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