The National Academies report noted that drawing a causal inference is "always complicated and, in the behavioral and social sciences, fraught with uncertainty."
Charles F. Wellford, chair of the committees that authored the report and a professor of criminology at the University of Maryland, says it’s the causal relationship that people are interested in when the question of guns and crime is broached. "While scientists can make the distinction between association and causation, in the real world the interest is in the latter," Wellford tells FactCheck.org, noting that this is his opinion, not the panel’s. "Work that knowingly reports findings that do not meet a causal test knowing they will be used as if they do can only produce confusion especially in such contentious issues."
The report said that "case-control studies" (the urban-area-to-urban-area type of comparisons) "show that violence is positively associated with firearms ownership." What the National Academies calls "ecological studies" (those comparing large areas, such as countries) "provide contradictory evidence on violence and firearms." But neither have shown a causal relationship. Both studies fail to address the multiple factors involved in the decision to buy a gun – owning a gun is not a random decision, said the report. And data on gun ownership may be insufficient (such numbers are based on surveys). It also faulted ecological studies that look at large geographic areas, saying, "there is no way of knowing whether the homicides or suicides occurred in the same areas in which the firearms are owned."
In comparing the United States to industrialized democracies, the Academies says data show the U.S. has the highest rate of homicide and firearm-related homicide. But this also raises a chicken-and-egg question. "A high level of violence may be a cause of a high level of firearms availability instead of the other way around."