Talk about timing.

A month after the massacre in Littleton, Colo., and not even a week after six students were shot at a Georgia high school, economist John R. Lott Jr. of the University of Chicago appeared at a conference to argue that there are good reasons for more people to have more guns.

Specifically, Lott and colleague William M. Landes, a law school professor at Chicago, say their research has led them to conclude that allowing people to carry concealed weapons dramatically reduces the occurrence and the severity of multiple-victim shootings.

Most experts find those assertions counterintuitive. Some dismiss them as ridiculous. As you might expect, gun advocates embrace Lott and Landes's findings, and gun opponents pillory them. In reply, Lott says: "Just look at the evidence"--which is exactly what his doubters and critics say they're doing.

Lott and Landes base their claims on a statistical analysis of multiple-victim shootings that occurred between 1977 and 1995 in the United States. They focused on shootings in public places "where the point of the attack was to produce carnage," Lott told a conference at the American Enterprise Institute here.

The researchers examined 396 attacks in which two or more people were shot and either killed or injured. They excluded shootings that occurred during robberies, drug deals and other crimes. Then they compared multiple-victim shootings in states that have "right-to-carry" weapons laws with those in states that didn't. They also conducted "before and after" studies to see what happened in states that relaxed their concealed weapons laws during the study period.

The states that enacted concealed weapons laws experienced "a dramatic drop in the number of people shot from multiple-victim public shootings," Lott asserted to the AEI audience. "The average number of deaths from shootings per 100,000 falls by 90 percent. The decline in injury rates from these shootings falls by 82 percent." (In raw numbers, the researchers estimate that allowing citizens to carry concealed weapons saved on average about seven lives a year per state and reduced the number injured by about the same amount.)

According to Lott, concealed weapons laws are the only type of legislation that appear to deter mass murderers or limit the damage they do. Background checks, more punitive sentencing practices and longer waiting periods to buy weapons seem to have little impact on preventing spree killings, he contends.

Why would fewer people die in massacres when people are allowed to carry concealed weapons? Lott doesn't know for sure, but he speculates that some would-be mass murderers may be dissuaded by the realization that they may be cut down before they commit enough carnage to get the notoriety they crave. Also, some killers may be captured or killed before they can finish their destructive rampage. (This happened after a 1997 school shooting in Mississippi, when an assistant principal retrieved his pistol from his pickup truck and apprehended the shooter as he tried to drive away.)

Thirty-one states, including Virginia, now have nondiscretionary "shall-issue" laws, which allow people to carry concealed weapons if they meet certain standards. (The requirements vary, but in most states, applicants must pass a background check and complete a gun safety course.) Another 12 states, including Maryland, have adopted "may-issue" laws, which require an individual to demonstrate a need to carry a weapon before getting a permit to carry a hidden handgun. Seven states and the District of Columbia prohibit concealed weapons.

Lott is no gun nut, though he's been called one (and called worse) by his critics. He's served as the chief economist for the U.S. Sentencing Commission and will begin teaching at Yale Law School this fall. His latest book, on enforcement of antitrust laws, will be published soon.

His 1998 book, "More Guns, Less Crime," put him at the epicenter of the gun debate. And he does have detractors in academe and elsewhere, including one who was invited to comment on Lott's research at the AEI conference.

Jens Ludwig, assistant professor of public policy at Georgetown University, argued that Lott's data don't prove "anything about what laws do to crime." He noted that crime rates, including homicide, are cyclical: They rise and fall every five to 10 years or so in response to forces that are not well understood.

Ludwig suggested that this pattern explains the apparent effectiveness of concealed weapons laws. Imagine, he said, a state where the murder cycle is on the upswing and approaching its peak and public concern is correspondingly high. Then a particularly ghastly mass shooting occurs. Panicked legislators respond by passing a law that allows equally panicked citizens to carry concealed weapons. A year or two later, the murder rate goes down, as Lott's study found.

One problem, Ludwig told the crowd at AEI: The murder rate would have drifted down even without the law. That's exactly what happened in Oklahoma, Kansas, Alaska and Louisiana after legislators left in place a ban on concealed weapons despite several mass murders that occurred in each state in a single year. His analysis of Lott's data suggests that it's just a coincidence that concealed weapons laws seem to be followed by a drop in mass murders.

Besides, allowing people to carry hidden firearms increases the chances that someone will misuse a gun, Ludwig said. Moreover, it increases the number of guns in circulation--thus making it harder for police to keep them out of the hands of criminals. And remember, Ludwig said: Relatively few people die in tragic but highly publicized massacres. Thus the cure--more guns in the hands of more people--may be worse than the disease.

Wrong and wrong, Lott shot back. He said his study took into account the so-called "murder cycle." And, he replied, "there's no evidence at all that an increase in gun ownership leads to an increase in homicide," or other types of crime.

It's a debate that wasn't resolved at the two-hour conference at a Washington think tank. It is, however, a debate worth having. It forces both sides in the familiar argument over constitutional rights to confront a more fundamental issue: What are the real costs and benefits to society of having so many guns in so many hands?