“By decreasing the extent to which people use e-cigarettes, you decrease quitting of conventional cigarettes,” said W. Kip Viscusi, a professor of law, economics and management at Vanderbilt University, who was not involved in the research but has studied tobacco policy extensively.

The research was conducted by Henry Saffer, Michael Grossman, Daniel L. Dench and Dhaval M. Dave, who used data from a detailed census survey about tobacco use to measure what happened to the smoking rate. Their goal was to find out whether e-cigarettes helped adult smokers quit smoking cigarettes, which are linked to a wide range of illnesses and are estimated to contribute to one in five deaths in the United States.

It’s possible, they figured, that vaping might encourage more people to smoke, by providing a new way to try nicotine for the first time. It might also cause people who might have quit to just keep smoking, by providing a second way to get nicotine where smoking is restricted. The natural experiment of the Minnesota tax helped them measure what some overall effects really were.

When Minnesota made vaping more expensive, they found, smokers kept smoking instead of switching to e-cigarettes. A longstanding decline in adult smoking in the state slowed way down, while smoking in states that hadn’t imposed big vaping taxes continued to fall. The researchers concluded that making e-cigarettes more expensive discouraged Minnesota smokers from trying them and caused fewer of them to switch away from smoking. By measuring the difference in the trends, the researchers estimated that Minnesota caused around 32,000 more adults to keep smoking cigarettes.

The paper didn’t include close measures of whether people who stopped smoking completely quit nicotine, the most healthful possible outcome for smokers. While it is clear that most vaping products are safer than cigarettes, it is not yet clear by how much. New research is emerging that vaping products may cause some long-term lung and heart disease. And a recent poisoning outbreak associated mainly with THC, in which 55 people died, suggests that there can be acute health risks for some users.