By John R. Lott Jr.

Everyone wants to keep criminals from getting guns. However, expanded background checks are not the simple answer that Sens. Pat Toomey (R., Pa.) and Joe Manchin (D., W.Va.) think they are.

Unfortunately, as the Senate considers the Manchin-Toomey amendment, Toomey is simply wrong to assert: "It's the people who fail a criminal or mental-health background check who we don't want having guns."

Toomey apparently does not understand how the background-check system works. Take his claim on Sunday: "Since checks began in 1998, more than 100,000 people who are ineligible to own guns have been denied them each year." Just because someone is "initially denied" permission to buy a gun doesn't mean that he is really ineligible to own guns.

This is the same problem experienced with the "no-fly" list. Remember the five times that the late Sen. Ted Kennedy was "initially denied" access to flights because his name was on the antiterror list? His name was similar to someone whom we really did want to keep from flying. But, by Toomey's method of counting, stopping Kennedy from boarding a flight means the "no-fly" list stopped terrorists from boarding a flight five times.

For gun purchases, the Bureau of Alcohol, Tobacco, Firearms, and Explosives later cleared more than 94 percent of those "initial denials." As the annual National Instant Criminal Background Check System report explains, these cases were dropped either because the additional information showed that the wrong people had been stopped, or because the covered offenses were so many decades old that the government decided not to prosecute. At least a fifth of the remaining 6 percent flagged were still false positives.

These initial denials mean delays for many law-abiding gun buyers. While a mere inconvenience for most, initial denials can pose a danger to people who suddenly and legitimately need a gun for self-defense - women being stalked by an ex-boyfriend or spouse, for example.

The numbers for 2010, the last year available, are instructive. With 76,142 denials, prosecutions were justified in only 42 cases, and there were only 13 convictions. Even those 13 cases don't tend to be the "dangerous" criminals Toomey claims are being stopped.

The limited number of prosecutions is not, as President Obama's critics claim, a matter of a new administration refusing to prosecute criminals. There were also few convictions under Presidents George W. Bush and Bill Clinton, and the reason was exactly the same.

Of course, background checks could possibly still stop a few crimes, even if almost no criminals try going through the background checks. As Toomey notes: "Determined criminals can find ways to obtain weapons. But background checks are helpful."

Undoubtedly, some criminals are stopped from getting guns. Yet there is a good reason that neither Toomey nor the president ever cites evidence showing that background checks have reduced crime. There is no such evidence. A 2004 National Academy of Sciences panel concluded that the Brady background checks didn't reduce any type of violent crime. Nor have later studies found a beneficial effect.

But what if the checks did reduce crime? Everyone would benefit, not just those legally buying guns. In that case, everyone, not just gun owners, should pay for the cost of running background checks. Unfortunately, the checks being considered by the Senate make only gun buyers pay the costs.

The delays in the background checks discourage gun ownership in other ways. States that have applied background checks to sales by private individuals have seen around a 20 percent drop in the number of gun shows, eliminating for many poorer people a relatively inexpensive source of guns. For gun shows, which usually last only a couple of days, even the two-day delay proposed by Manchin and Toomey means that no sale would be made.

The Manchin-Toomey amendment does include some small provisions that gun owners will appreciate. People will be able to buy guns outside their home states. And veterans who have suffered from post-traumatic stress disorder would be better able to restore their rights to own a gun. But these gains are small compared with the legislation's costs.

Expanded background checks might intuitively seem to make sense. But how laws work in theory is often different from how they work in the real world. Unless the databases somehow are dramatically improved, expanded background checks are likely to do more harm than good.

John Lott is a former chief economist at the U.S. Sentencing Commission and the author of the recently released book "At the Brink: Will Obama Push Us Over the Edge?" (Regnery).