Daily smoking among 10th-graders was cut 20.8 percent when enforcement of laws against tobacco sales to underage youth were stepped up in response to a law tying federal aid to compliance rates, according to a new study.

Researcher Joseph DiFranza, M.D., of the University of Massachusetts Medical School led a team that examined national data on enforcement of tobacco-sales laws in the seven years after Congress passed the so-called Synar Amendment, which set benchmarks for retailer compliance to compel state and local sting operations aimed at underage sales.

The study concluded that for every 1 percent increase in the merchant-compliance rate, daily smoking among 10th-graders fell 2 percent. Higher cigarette prices, bans on indoor smoking, and anti-tobacco advertising also contributed to overall declines in teen smoking reported between 1997 and 2003, the researchers said.

“Skeptics argued that prohibiting sales to minors wouldn’t help, because kids would always be able to get cigarettes somewhere,” said DiFranza. “Our data suggest that a 25-percent increase in compliance of laws prohibiting cigarette sales to minors has about the same deterrent effect as increasing the price by $2 in 2006 dollars … This study clearly demonstrates that enforcing these laws reduces smoking rates among youth.”

DiFranza said the findings could help convince other countries to step up enforcement of their own laws governing sales of tobacco to youth.

The study was published April 17, 2009 in the journal BMC Public Health, and funded by the Substance Abuse Research Policy Program (SAPRP) of the Robert Wood Johnson Foundation.