The very laws restricting teenagers from using e-cigarettes are pushing them toward traditional ones, research shows.

Most states’ minimum age for e-cigarette sales and other tobacco products is 18, though two states and 225 localities (such as Washington, D.C. and New York City) require buyers be 21 years old for all tobacco products. Since states began instating e-cigarette laws, though, starting in 2010, participation of youth smoking of traditional cigarettes jumped from 0.7 to 1.4 percentage points, according to a new paper by Dhaval Dave, an economics professor at Bentley University, Bo Feng, a doctoral student at Georgia State University, and Michael Pesko, a health economist and assistant professor of health-care policy and research at Weill Cornell Medical College.

By making it harder for people under 18 to buy e-cigarettes, which are considered vaping products and have less nicotine, the researchers argue teenagers are instead turning to traditional cigarettes, which are easier to obtain and less costly, Pesko said. Because vaping is more popular among youth than older adults, teenagers might be able to get traditional cigarettes from other outlets, such as their parents, whether voluntarily or by stealing, he added. E-cigarettes also become more expensive on the underground market because of the hassle of getting them, even though they’re less expensive when sold in stores compared with traditional cigarettes.

“When you make it harder to buy e-cigarettes, the unintended consequence of this action is continuing regular cigarette consumption,” Pesko said. The laws didn’t, however, impact use of marijuana use or alcohol consumption.

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Across the country, e-cigarettes have grown in popularity, especially for youngsters, going so far as to pass traditional cigarettes in commonly used tobacco products. Between 2011 and 2015, e-cigarette use has jumped from 1.5% to 16% among high-school students, and 0.6% to 5.3% among middle-school students, according to another report.

E-cigarettes, which are a vaping device with less nicotine than traditional cigarettes, were first introduced in 2007 and were under the watchful eye of the government last year. The U.S. Food and Drug Administration (FDA) placed a ban on all tobacco product sales to minors and required tobacco manufacturers, including e-cigarette vendors, to submit products for premarket review. The agency took its decision seriously, and issued 24 letters to websites selling e-cigarettes to minors illegally within the first month of implementing the regulation.

Of course, there are risks associated with e-cigarettes. They still have nicotine (though about 5% the amount of a traditional cigarette, Pesko said), which could lead to addiction and harm brain development, the Centers for Disease Control and Prevention wrote on its site. Other risks include mood disorders and lack of attention and cognition, the U.S. Surgeon General noted in his report in December. Yet, youth, including middle school and high-school students, are faced with e-cigarette advertisements in stores online and on television — more than 18 million youth were exposed to these ads in 2014. Meanwhile, the U.S. is seeing a decline in the number of traditional cigarette smokers — about 15% of the adult population smoke cigarettes now, compared with 42% who did so in 1965, the CDC reported.

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States are taking initiatives, such as considering pushing up the minimum age of tobacco product sales, in an attempt to reduce smoking and tobacco use among youth. Hawaii and California have already raised the age, and other states considering the move include Massachusetts, New Jersey and Washington. In their paper, Dave, Feng and Pesko argue regulators may want to increase the minimum age requirement of traditional cigarettes to 21 years old, but keep e-cigarettes at 18, which could help youth quit smoking traditional cigarettes. “Preventing them from legally buying e-cigarettes until age 21 may harden preferences for cigarettes and make quitting at that age more difficult,” they wrote.