Enforcement of laws banning tobacco sales to minors has curbed U.S. teenagers' smoking rates, a new study finds.

Between 1997 and 2003, merchants' increasing compliance with tobacco- sales laws led to a 21 percent reduction in the number of 10th-graders who were daily smokers, researchers report in the online journal BMC Public Health.

The decline followed a 1996 federal law that compelled states to crack down on tobacco sales to minors — including having local authorities send underage "decoy" shoppers into stores to try to buy cigarettes.

Over the same 1997-2003 time period, the study found, increasing tobacco prices also seemed to deter teens from smoking. For each dollar increase in tobacco prices, the odds of a teenager being a daily smoker dropped by half.

"Our data indicate that improving merchant compliance with the prohibition on sales of tobacco to minors and increasing the price of cigarettes discourage youth smoking," write Dr. Joseph R. DiFranza and his colleagues at the University of Massachusetts Medical School in Worcester.

Before the 1996 legislation, known as the Synar Amendment, tobacco sales to minors were prohibited, but many retailers failed to comply.

"There is no doubt," DiFranza and his colleagues note, "that the Synar Amendment with its threat of financial penalties for states was the prime motivator for most states to enforce their laws."

This is the first study, they write, to show that the law has had its intended effect on the national level.

While higher prices seemed to be an even greater obstacle to teen smoking, the researchers add, "there is no reason why policy makers should choose between these approaches, as all effective measures to reduce smoking among youth should be employed."