There is a little quasi-statistical mystery deep inside the gun-control debate, but no one ever seems to mention it. We say, ban the guns that kill our children. The gun lobby says, relax gun laws so that we can arm the good guys; "the only thing that can stop a bad guy with a gun is a good guy with a gun."

According to Mother Jones magazine, in the 62 mass shootings that have occurred in the United States during the past 30 years, never — not once — has an armed civilian halted an attack by firing on a perpetrator. Oddly, Mother Jones did not ask the logical follow-up questions: Why not? Was no one carrying a gun ever present at one of these shootings? That seems unlikely. And yet both sides of the gun-control debate seem to be operating on the premise that people have not already been carrying guns in places where mass shootings happen. Both common sense and my own personal experience as a criminal lawyer belie that assumption. The available statistics certainly suggest that the number of guns and gun-owners in the country is so high that it seems unlikely that the mass shooter was the only one there who had a gun. One study estimated that there are 88.7 guns for every hundred Americans. As for gun owners, various "experts" have suggested they represent anywhere from 34 percent to 47 percent of the population.

If we are going to try to calculate the statistical likelihood of a "good guy with a gun" having been present at a mass shooting, we need to consider the number of people nearby at the time of the violence. The 62 mass shootings took place in workplaces, schools, movie theatres, shopping malls, government buildings, places of worship, restaurants and cafes. Although we have no statistics on how many people were there at each incident, doesn’t common sense give us a ballpark answer? Judge from your own experience: When you have gone to the shopping mall or to the multiplex, how many people were present or in the vicinity? And yet, during a 30-year period in which there were 62 mass shootings, there has never been so much as one report of a good guy stopping a mass shooting by taking out the shooter. Given what we know about gun ownership in this country (to say nothing of the frequency of illegal possession of weapons in this country), does that figure not strike you as statistically unlikely?

There is, of course, another possible explanation, which is that there have been "good guys with guns" present at prior shootings. If you think about it, there are some pretty good reasons for a gun-carrying civilian not to return fire. First, he may simply panic. Second, the bystander may assess the attacker’s firepower against his own and decide he would not stand a chance. Third, he might quite sensibly decide not to use his gun because a gunfight might simply increase the carnage. In all three scenarios the good guy with a gun does not stop the bad guy with a gun. (Indeed, even when armed guards have been present, as at Columbine and Virginia Tech, they failed to thwart the massacres.)

And what happens to the gun lobby’s argument — the claim that the only way to stop bad guys with guns is to arm good guys — if we discover there have been good guys with guns present at the scene of mass shootings — and they did not, in fact, draw their weapons? That pretty much shoots it down, don’t you think?

Peter L. Davis, formerly a public defender in New York, is associate professor of law at Touro College Jacob D. Fuchsberg Law Center in Central Islip, N.Y.