Approximately 9 million handgun owners in the United States carry loaded handguns on a monthly basis—about 7 million of whom have concealed carry permits—while 3 million report carrying on a daily basis.

That's according to a study led by Northeastern professor Matthew Miller and his colleagues, and published in the American Journal of Public Health.

Miller, professor of health sciences and epidemiology, helped design the overarching survey that he and his colleagues used in the current study of 1,444 handgun owners. His collaborators included researchers from the University of Washington School of Public Health, the University of Colorado-Denver, and the Harvard School of Public Health.

Data from the survey showed gun owners who carry are most likely to be male, aged 18-29, conservative, and from the South. Factors of race, income, education, and veteran status were not noticeably different in gun owners who carried compared with those who didn’t. And the vast majority of people carrying a weapon cited protection as the reason for doing so.

Carrying behavior varies across states depending on their concealed carry laws, the study found. As one might expect, more gun owners carried in states that exercise less discretion in granting concealed carry permits. Proportionately fewer gun owners carried in states where permitting requirements were more exacting.

Overall, state laws regulating concealed carry have become more lenient over the past couple of decades. Many places used to allow the police and other authorities to review applications and make decisions about issuing permits. But that’s no longer common practice, Miller said, which has made permits easier to get. In previous research, Miller has found that one in five U.S. gun owners who obtained a firearm in the past two years did so without a background check.

In addition, the estimated number of privately-owned guns in America grew by more than 70 million—to approximately 265 million—between 1994 and 2015, and half of that gun stock is owned by only 3 percent of the population, based on the findings last year of a comprehensive national survey that Miller co-led.

Miller said the findings point to a need for more research in this area to better understand the public health consequences of carrying firearms in public places. But research on firearms is difficult to pursue because of a lack of funding. “With pressure from the National Rifle Association and other pro-gun lobbyists, government agencies grant very little money to study guns and their impact on society,” Miller said.

“There is no credible evidence to suggest that allowing people to carry has any beneficial effect on crime and violence,” Miller said. “And although some recent studies suggest there may be harmful effects, there is a lot more we need to know before we can reliably place in perspective what those harmful effects are.”