(CNN) -- States with the most gun laws experienced a lower overall mortality rate from firearms than did states with the fewest laws, researchers in Boston reported in a study published Wednesday.
"States that have the most laws have a 42% decreased rate of firearm fatalities compared to those with the least laws," said Dr. Eric W. Fleegler, an attending physician in pediatric emergency medicine at Boston Children's Hospital and an assistant professor of pediatrics at Harvard Medical School.
Those states with the most gun laws saw a 40% reduction in firearm-related homicides and a 37% reduction in firearm-related suicides, he said in a telephone interview.
Fleegler, the lead author in the study published online in JAMA Internal Medicine, reached that conclusion by analyzing data reported to the federal Centers for Disease Control and Prevention from 2007 through 2010 and then correlating those figures with state-level firearm legislation aggregated by the Brady Center to Prevent Gun Violence.
Figuring out how many laws existed in each state was difficult. "What do you do when one law has seven parts" Fleegler asked. "Is that seven laws? Is that one law?"
So the researchers checked the state laws to determine whether they were intended to curb firearm trafficking; strengthen background checks beyond what's required under the Brady Handgun Violence Prevention Act; ensure child safety; ban military-style assault weapons; or restrict guns in public places.
Based on how many of those categories a state's laws covered, the researchers calculated a "legislative strength score," which they compared with firearm-related mortality rates in all 50 states. The legislative strength scores ranged from 0 in Utah to 24 out of a possible 28 in Massachusetts.
Over the four years scrutinized, 121,084 firearm fatalities occurred, with rates ranging from a high of 17.9 per 100,000 in Louisiana to a low of 2.9 per 100,000 in Hawaii.
When compared with the quartile of states with the fewest laws, the quartile of states that had the most laws had a lower firearm suicide rate and a lower firearm homicide rate, Fleegler said. The absolute difference in the suicide rates was 6.25 deaths per 100,000; in the homicide rates it was 0.40 deaths per 100,000.
"When you're talking about 300 million people, you're talking about thousands of deaths that would not otherwise have occurred," Fleegler said.
Even on a state level, some figures were striking. For example, there was a three-fold difference in firearm-related suicide between Massachusetts and Louisiana, which has few laws limiting the use of firearms.
"We anticipated that there was going to be a relationship between state laws and firearm mortality," he said. "The magnitude of the effect, a 42% reduction, that was a big number to look at."
The authors acknowledged that they showed only an association; they did not prove that more laws on firearms translate into fewer deaths.
Fleegler said his study "speaks to the importance of having legislation. One of the things that we've learned over time is that there are laws that have been passed that have large loopholes, and those loopholes make the enforcement and efficacy of the laws diminished. There are ways to make these laws better and stronger."
But Dr. Garen Wintemute, an emergency medicine physician and director of the Violence Prevention Research Program at the University of California, Davis, urged caution in interpreting the study in an accompanying editorial published in the journal.
"Correlation does not imply causation," he wrote. "This fundamental limitation is beyond the power of the authors to redress."
He added that the list of laws takes no account of differences between states in the specifics of laws and takes no account of how hard states worked to enforce those laws.
The biggest difficulty, Wintemute continued, is that almost all of the associations between more laws and fewer deaths disappeared when the investigators took into account the prevalence of gun ownership in each state.
"This is a problem because there are two completely opposite explanations for why that might be the case," Wintemute said in a video issued by his university. "One is that these laws work, and that they work by decreasing the rate of gun ownership in a state, because we know that the rate of gun ownership is associated with the rate of violent death in a state.
"But the other possibility, that's at least as plausible, is that it's easier to enact these laws in states that have a low rate of gun ownership to begin with. Gun ownership is not as important in those states, there's less opposition."
He added, "We really don't know what to do with the results. We cannot say that these laws -- individually or in aggregate -- drive firearm death rates up or down."
He predicted that policy makers would not be able to draw useful conclusions from the work. "The conclusion that I draw is we need to get this question answered right."
Wintemute said the researchers did a good job with the limited data they had available but said the larger problem dates back to the 1990s, when the National Rifle Association inserted language into the CDC's appropriation that limited its work on how to reduce firearm injuries.
Now, as lawmakers are looking for evidence on what works, "investigators like this group are reduced to doing the best they can with what's available," he said.
For his part, Fleegler bemoaned the dearth of data from individual cities about firearms-related injuries and noted that data on enforcement of those laws were also spotty. "We agree that there is a lot more research that needs to be done, that funding to allow robust research and robust collection of data is what's really going to move the science forward for understanding how we can reduce deaths," he said.