KANSAS CITY, Mo. >> Jean Peters Baker, the county prosecutor here, stood at her desk poring over Facebook photographs of young men posing with guns. One wore a grinning mask and pointed his gun at the camera. Another clasped guns in each hand. A third was laughing uproariously, his finger on the trigger of an assault rifle.

“This is our reality,” Peters Baker said, gesturing toward the pictures from recent investigations. “I’m not talking about my uncle who still lives on a farm in central Missouri and uses a gun for hunting.”

In the past decade, Missouri has been a natural experiment in what happens when a state relaxes its gun control laws. For decades, it had one of the nation’s strongest measures to keep guns from dangerous people: a requirement that all handgun buyers get a gun permit by undergoing a background check in person at a sheriff’s office.

But the legislature repealed that in 2007 and approved a flurry of other changes, including, last year, lowering the legal age to carry a concealed gun to 19. What has followed may help answer a central question of the gun control debate: Does allowing people to more easily obtain guns make society safer or more dangerous?

The mass shooting in San Bernardino, Calif., this month has reignited that debate in America, and Missouri’s experience offers one perspective. It is difficult to isolate the impact of gun laws in a single state, given the pervasiveness of interstate trafficking and illegal markets, but a variety of measures, including a marked increase in police seizures of guns bought in-state, suggest the changes in Missouri’s laws have had some effect.

Research by Daniel Webster, director of the Johns Hopkins Center for Gun Policy and Research, found that in the first six years after the state repealed the requirement for comprehensive background checks and purchase permits, the gun homicide rate was 16 percent higher than it was the six years before. During the same period, the national rate declined by 11 percent. After Webster controlled for poverty and other factors that could influence the homicide rate, and took into account homicide rates in other states, the result was slightly higher, rising by 18 percent in Missouri.

Federal death data released this month for 2014 showed a continuation of the trend, he said. Before the repeal, from 1999 to 2006, Missouri’s gun homicide rate was 13.8 percent higher than the national rate. From 2008 to 2014, it was 47 percent higher. (The new data also showed that the national death rate from guns was equal to that of motor vehicle crashes for the first time since the government began systematically tracking it.)

Other measures suggested that criminals had easier access to guns after the permit law was repealed. Webster analyzed data from the federal Bureau of Alcohol, Tobacco, Firearms and Explosives, and found that the share of guns that were linked to crimes soon after they were bought doubled in the state from 2006 to 2010. The portion of guns confiscated by the police in Missouri that had been originally bought in the state — ordinarily a very stable statistic — rose to 74 percent last year, from 56 percent before the law changed.

In interviews, researchers cautioned that causation is hard to prove, and that just because the gun homicide rate rose after 2007, it does not mean the repeal was the reason. Still, most of them were convinced that the data suggested an effect.

A few were not. Gary Kleck, a professor of criminology at Florida State University in Tallahassee, said he doubted that Missouri’s permit law had ever blocked many criminals from getting guns. Gun homicides in California rose after the state banned one category of guns, so-called junk guns, he said, suggesting tighter laws were not safer. Webster noted that the rise in California disappeared with more years of data.

Opponents point out that California, where the San Bernardino gun attacks happened, has some of the strictest laws in the country. But supporters say that mass shootings, while attention-grabbing, account for fewer than 2 percent of the more than 30,000 gun deaths in the United States each year. They say tougher gun laws help reduce the slow, steady stream of killings that pile up quietly in communities like this one, often poor, often of color, and cut down on suicides, which make up two-thirds of all gun deaths in the United States.

Rigorous scientific research on universal background checks is sparse, in part because federal funding for it is practically nonexistent. A number of states toughened their laws after the 2012 mass shooting at an elementary school in Newtown, Connecticut, but the changes were too recent to evaluate the effects.

Missouri was the only state in recent history to repeal a law requiring background checks and permits for all handgun sales, and Webster said he had been drawn to study the aftermath because many had considered that type of law to be the most effective at keeping guns from people who should not have them. In 1995 Connecticut enacted a law similar to the one Missouri repealed, and gun homicides declined by 40 percent in the 10 years that followed, he found.

A small group of public health experts who study firearms — including Garen J. Wintemute of the University of California, Davis; Philip J. Cook of Duke University; and Webster — recently received private grants to begin evaluating the effect of such changes in other states.

Missouri began changing its gun laws after the Republican Party won control of the state House in 2002 for the first time in years. But many Democrats also supported relaxing the restrictions.

The changes tapped into profound differences between rural and urban Americans about guns. The state Legislature is predominantly white, rural and suburban, but the effects of the laws it makes are felt largely in Missouri’s cities, where gun homicides are one of the biggest causes of death for young black men.

In Webster’s analysis, the gun homicide rate rose by 20 percent in metropolitan areas of Missouri, but was up by just 1.6 percent in rural areas. However, gun suicides, largely a rural, white problem, rose by about 16 percent in the years after the repeal, he found.

Kansas City’s mayor, Sly James Jr., said he had argued against the gun control changes, but to no avail. He called gun homicides in his city “slow-motion mass murder.”

“I’m a black mayor in a state with a rural representative conservative legislature that gives the rights to carry arms to 19-year-olds — do you really think there’s anything I can say that’s going to bother them?” James said.

“I’m tired of the fact that we have disproportionate numbers of African-Americans who are dying,” he said. “Legislators, who are predominantly white, seem to ignore the racial implications of the laws they make.”

Local political leaders who support gun rights say criminals will get their hands on guns no matter what the law says. The legal changes protect the rights of law-abiding citizens to own guns, they say. To that end, state Sen. Kurt Schaefer, a Republican, sponsored an amendment to the Missouri Constitution that declared gun ownership an inalienable right.

The amendment, which took effect last year despite opposition from prosecutors, has thrown some laws into question. For example, felons have recently challenged gun charges against them, claiming the amendment makes their right to own a gun “inalienable,” even though state law forbids it.

“Yes, there is a crime problem and, yes, some of those crimes are committed with guns,” Schaefer said. “But taking guns away from law-abiding citizens — that does not solve the problem.”

Crime experts in the state see the problem differently.

“There is this idea that law-abiding citizens’ rights are being secured,” said Richard Rosenfeld, a professor of criminology at the University of Missouri, St. Louis. “In fact, it’s the people most inclined to do harm whose privileges are being secured.”

As Rosilyn Temple tells it, guns have become so prevalent in her Kansas City neighborhood that owning one is about as common as owning a cellphone. “You can’t buy liquor, but you can carry a gun,” she said, alluding to the legal age to obtain a concealed carry permit. The legal age to buy alcohol in Missouri is 21.

In 2011, the day before Thanksgiving, Temple’s son was shot to death in his apartment. She had not heard from him all afternoon that day, and after taking her sweet potato pies out of the oven, she pulled on her boots and coat to go check on him. Finding his body broke her heart and split her life in two — the before and the after.

Now, she is an advocate for mothers who have lost children to violence, calling dozens of them a week, helping with funeral arrangements and explaining the police to them and them to the police.

Valerie Dent, 62, a retired factory worker in St. Louis, helps make those calls in her city. Her sons, James and Steven, were shot and killed one morning last fall after coming home from working the night shift at a talcum powder factory. Gun violence, she said, has done more to grind up black families, like hers, than any kind of aggressive policing. As with Temple’s son’s case, her sons’ murders are unsolved.

“It becomes a way of life — to hear gunshots and close the door,” Dent said. “We get all in an uproar when the police kill someone. If we had that get-up-and-go on the black-on-black crimes, maybe we could stop some of this.”

Of all the gun law changes, the one that most affected homicides was the repeal of the permit law requiring a background check at a sheriff’s office, Webster argued. It had applied to any prospective gun buyer — even someone buying from a private seller.

Now permits are no longer required. Gun buyers must still pass background checks if they buy guns in stores, which send applicants’ personal information to the FBI. Gun control proponents say those checks are less rigorous than before, though gun rights supporters insist they are just as thorough. Buying from a private seller requires no check at all.

Some gun rights supporters say the change was less significant than it appeared because people buying from private sellers rarely bothered with permits even under the old system.

“A lot of people didn’t even know there was a permit system,” said Kevin L. Jamison, a lawyer in Missouri who concentrates on weapons cases.

Webster said there was no way to tell for sure that the old system worked, because there was no central repository for permits, but that the rise in crime guns originally purchased in Missouri suggests that the previous law was effective.

The changes have complicated the daily work of the police. Col. D. Samuel Dotson III, the police chief in St. Louis, said a recent change that made it legal to carry a firearm in a vehicle without a permit had made it harder for officers to connect an illegal gun to a person during a traffic stop. Rosenfeld, of the University of Missouri, analyzed hundreds of arrests involving guns from 2011 and found that 40 percent of the cases were never prosecuted, because the change made it harder to prove the charges.

Guns confer status. And because the repercussions for carrying them without the proper authorization are so minor, there is little risk in showing them off.

“The big thing used to be having a fancy car and driving it around,” Dotson said. “Now, it’s having a pistol with extended magazine and posting pictures of it on Facebook.”

Jennifer M. Joyce, the top prosecutor in St. Louis, keeps a framed picture of a teenager from one of her recent cases posing with a gun.

“It has reached the point in St. Louis where this guy can get a gun really easily,” she said. “He believes that gun is going to allow him to avenge the death of his buddy. But what we all know is that gun will either be the death of him or cause him to end up in prison.”

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