(CNN) The real effects of most gun laws are unknown due to a scarcity of evidence, a new and far-reaching systematic review of US gun policy finds.

The analysis, which was intended to seed the gun debate with nonpartisan data, did, however, find "credible evidence" that child-access prevention laws reduce unintended firearm injuries to children. These laws, which require guns to be stored safely, also reduce self-injuries and suicides, say the researchers at RAND, a nonprofit research organization.

Access prevention laws reduce unintended firearm injuries to children and also reduce self-injuries and suicides, the evidence suggests, researchers at Rand found.

"We don't call it strong evidence, we call it supportive evidence," said Andrew Morral, the project's leader and a behavioral scientist at Rand. "It's not like the level of evidence we have that smoking causes cancer, for instance."

The effects of these same laws on defensive gun use -- when a crime victim shows or uses a firearm in self-defense -- remain unknown. Crucial missing evidence results in an incomplete portrait of what might happen if access prevention laws were to be passed throughout the nation.

If there's been an impasse for decades and the impasse "turns on matters of fact that could be resolved," said Morral, then it makes sense to do more research, find the facts and resolve the issue.

'Not well-studied'

The passionate clash over gun policy served as an inspiration for Morral's two years of study. The existing "uneven research" and "contradictory studies" mean "the two sides can then cherry pick studies that happen to support what they already believe," he said.

To find reliable information about the effects of different gun laws, Morral and his colleagues looked at thousands of studies and then winnowed it down to those that actually say something about what a specific gun policy may cause once it is enacted. Morral and his colleagues also conducted a survey of gun policy experts to understand where there are key disagreements on factual matters.

Two big conclusions can be drawn from the results of the new research, he said.

"Most of the kinds of facts that people care about in these gun policy debates have not been well-studied," said Morral. "So we can't really say with precision what the effects of the gun policy would be on a whole range of things."

Defensive gun use, for example, is a concern to gun owners. "We found almost no research on how different policies might affect their ability to defend themselves," he said.

'A difference about facts'

A second major conclusion is that the source of disagreement is not due to different values or a desire to achieve different objectives, he said. "We found some pretty good evidence that the difference between these groups is really a difference about facts."

that allows you to explore the two different perspectives on what the effects of laws would be and how it might affect each state and the country," said Morral. "We created this online tool that allows you to explore the two different perspectives on what the effects of laws would be and how it might affect each state and the country," said Morral.

Moderate evidence supports establishing background checks to reduce gun suicides and homicides and prohibiting purchases of guns by people with mental illness to decrease violent crime, the report found.

"You can see some areas where there is very big differences in belief," said Morral, pointing to stand-your-ground laws, which have been passed in more than half the states. These laws protect the right to use a gun in self-defense when faced with a threat.

Experts who favor what Morral calls "permissive" gun laws -- those aligned with the National Rifle Association -- believe stand-your-ground laws, if turned on in all 50 states, would result in a 4% reduction in firearm homicides -- about 500 fewer homicides per year.

By contrast, experts who favor restrictive laws -- those aligned with the Brady Campaign to Stop Gun Violence -- believe stand-your-ground laws would result in a 2% increase in gun homicides or about 300 more per year.

One critic believes the new Rand report "rests on two fundamentally flawed concepts."

"First, that advocates on all sides have 'a disagreement about facts, not about values or objectives' and second, that research being conducted in the area of gun control is objective," said F. Paul Valone, president of Grass Roots North Carolina, a nonprofit gun rights organization.

The new report fails "to acknowledge the underlying and rarely stated objective of the gun control movement to end private gun ownership," said Valone, who was not involved in the research. It was biased studies that led to the congressional decision to defund gun injury research, he said.

Research that has been conducted on concealed handgun laws , for instance, has been discredited without proper assessment of merits and flaws, he said.

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"Preservation of constitutionally guaranteed freedoms is not antithetical to community safety" in the presence of "rational, fact-based public policy," said Vallone.

Heated though the argument may be, Morral believes both sides equally value the same goals.

"The number one objective for both groups is reducing firearm homicides, suicides, mass shootings and accidents," he said.