If the man who killed 12 people and shot dozens more in a packed Colorado movie theater last week had tried to carry out his scheme in a different state, or at an earlier time, he would have faced more obstacles.

The suspect, James E. Holmes, bought a semiautomatic assault rifle, two semiautomatic pistols and a 12-gauge shotgun at stores in Colorado and 6,000 rounds of ammunition online along with a 100-round magazine. He did so after easily obtaining a permit at a gun store, since he had no criminal record.

In California and Massachusetts, most assault rifles and large-capacity ammunition magazines are banned, as they were across the country from 1994 to 2004 by federal law. Before a 1986 change in the law, ammunition could not be legally sent by mail.

And if Holmes had tried to buy his guns in New Jersey, he would have had to apply to local law enforcement agencies or the state police — not to a gun shop — and answer questions about personal history, including whether he had ever been “observed by a psychiatrist” for “a mental condition,” even a temporary one, and to waive all confidentiality. His behavior, which some have described as erratic in recent months, might have raised concern.

“For people who are somewhat disturbed or behave oddly under stress, such an application process becomes another screen to weed them out,” said Paul S. Appelbaum, a professor of psychiatry, medicine and law at Columbia University.

Few issues — the Israeli-Palestinian conflict and climate change come to mind — have as extensive or tortured a history of entrenched disagreement as the relationship between violence and gun control laws does. Feuding scholarly traditions, each backed by statistical analysis, assert either that tighter laws reduce killing or have nothing to do with it.

And while gun ownership rates and gun restrictions have declined over the past 50 years, murder rates have risen and fallen. Today, with fewer gun control laws in place than 20 years ago, homicide rates are down.

What can be asserted is that the states with the strictest gun laws — Hawaii, Massachusetts, New Jersey, New York — have among the lowest gun death rates, according to figures from the federal Centers for Disease Prevention and Control, and those with the most lenient laws — Alabama, Alaska, Louisiana, Mississippi — have among the highest.

Whether there is any causal relationship between the two remains in dispute, as does whether more deaths can be attributed to the availability of more guns.

Philip J. Cook, a professor of public policy at Duke University, put it this way: “My research over 35 years demonstrates that the effect of gun availability is not to increase the crime rate but to intensify the crime that exists and convert assaults into murders. I have never seen evidence that gun access influences the volume of violent crime. But when you add guns to a violent situation, you get a higher level of murder.”

That may seem an unassailable conclusion: When angry or disturbed people are armed with, say, knives rather than guns, they cause fewer deaths. And indeed, Cook is not alone in drawing it. An extensive review of the scholarly literature by the Harvard Injury Control Research Center found that where there were more guns there was more homicide, both across the United States and throughout other high-income nations.

But Gary Kleck, a professor of criminal justice at Florida State University, believes that guns do not bring murder; murder brings guns.

“There is unanimous evidence that higher homicide rates lead to people getting more guns,” he said, in part because people arm themselves for protection. “But our statistical analysis finds no homicide effect of more guns.”

Asked about the lower homicide rates in states like New York and Massachusetts, he said that they had lower rates of violence before any gun control laws were put into effect.

“It’s like comparing the United States and England,” he said. “Neither had gun laws before about 1920, and England had a homicide rate about one-eighth of the United States’.”

Kleck does not disagree that in Massachusetts or New Jersey, faced with law enforcement screening, Holmes would have found it harder to get his gun permit. But he opposes such a system because he does not trust the police to decide who should own a gun.

“I am not comfortable with their deciding whether others can have guns or not,” he said. “Police think the ideal is for them to be armed and for the rest of us not to be. I don’t see it that way.”

Efforts to measure whether gun control laws reduce homicide have produced little. In fact, Cook of Duke said that he and a colleague examined the impact of the Brady Law, signed with great fanfare by President Bill Clinton in 1993 and requiring background checks before a firearm can be bought from a federally licensed dealer. They found virtually no reduction in homicide after the law was put into effect.

Cook said, however, that many guns are bought and sold outside that system, making the data hard to evaluate.

This is a point made by nearly all researchers, that gun laws have generally been so poorly enforced with so many loopholes that judging their effectiveness has been enormously challenging. Even if Holmes had been blocked from buying a specific model of assault rifle, in California, Massachusetts or New York state, he would most likely have been able to buy a similar gun not covered by the law.

Efforts to renew a federal ban on assault weapons or large-capacity ammunition magazines have gone nowhere. Americans have grown increasingly mistrustful of gun control, viewing it as ineffective.

Daniel W. Webster, co-director of the Johns Hopkins Center for Gun Policy and Research in Baltimore, still believes that the laws in a number of states have proved themselves.

He published a widely cited study in 2009 that found that states with strong gun dealer regulations and oversight as well as tight sale permits had less diversion of guns to criminals. States without such measures had more guns diverted to an illegal context, typically within a year of sale.

In a new study he plans to publish soon, he found that the same sorts of accountability measures helped prevent guns from going across state lines to criminals; states without such measures ended up exporting more guns into the hands of criminals.

“What keeps guns from criminals?” he asked. “Good gun control clearly does, and the lack of it facilitates diversion. All of the research shows that availability of guns is important. If a guy goes into a theater with a knife or a club, that is very different than if he goes in with a gun. Guns matter.”

© 2012 The New York Times Company