Justice Lab is a column that examines the science, social science and technology of criminal justice

After every mass shooting, there is a demand for gun control and then...nothing seems to happen (or, gun laws are loosened). That pattern dates back four decades, to the early 1970s, when rising crime rates inspired legislators to introduce gun bills at the state and federal level. The political compromise forged back then between the gun lobby and tough-on crime legislators — in which access to firearms is largely untouched, but prison sentences for gun crimes are lengthened — remains enshrined in federal and many state laws, even though social scientists debate whether these policies reduce crime.

The laws are referred to as gun “add-ons” or sentence “enhancements.” They require judges to increase a defendant’s prison term if he or she had a gun at the time of committing a crime such as robbery or drug dealing, even if no shots were fired, and sometimes even if the gun was not at the scene of the crime. In one infamous case, Weldon Angelos was caught selling marijuana to Salt Lake City police informants while possessing guns (there are conflicting accounts as to whether a gun was on his body at the time of the sales). He was convicted of three counts of drug trafficking while possessing a firearm, and sentenced, under mandatory gun minimums, to 55 years in prison. U.S. District Court Judge Paul Cassell, who presided over the case, called the required sentence “unjust, cruel, and even irrational.”

The Sentencing Reform and Corrections Act, introduced by a bipartisan group of senators on Oct. 1, is meant to decrease the federal prison population and rein in costs. It is being celebrated by advocates for criminal justice reform, who have focused on its promise of shorter prison sentences for nonviolent crimes. The bill would reduce tough gun add-ons for defendants like Angelos, who have multiple or prior convictions. If he had been sentenced under the new proposal, Angelos would have received a 15-year prison term instead of 55 years. But in a sign of how politically sacrosanct gun enhancements are, the bill maintains mandatory sentence add-ons of five to 30 years for first-time offenders convicted of possessing, brandishing, or firing a gun in the course of committing a crime.

There are three major arguments in favor of gun enhancements. First, the incapacitation effect of long prison sentences could prevent gun offenders from committing new crimes. Second, word of harsh sentence add-ons could spread to would-be criminals, who will then choose not to carry guns. And third, enhancements target gun owners who are dangerous, without making it more difficult for law-abiding gun owners to purchase or possess firearms.

In 1975, David Bartley and J. John Fox, Massachusetts state legislators who crafted one of the first such laws, wrote a New York Times op-ed making the case for gun add-ons. “The Massachusetts law is designed to remove the temptation to carry guns and therefore greatly reduce the chance that they will be used,” they wrote. “The gasoline station owner, the drug-store proprietor, the variety-store keeper, the doctor in a hospital, the executive or employee who works late at an office and has been the target of prowlers and thieves will all be able to legally keep their guns.”

But social science is divided on whether gun add-ons actually deter or prevent violence in the real world. A 2010 study conducted by David Abrams, an economist on the faculty of the University of Pennsylvania Law School, found that in the first three years after gun enhancements were enacted in 30 states, gun robberies declined by approximately five percent on average, and robberies without guns declined by 3 percent. The crime reduction was not likely due to increased policing, Abrams said, because each of the jurisdictions he studied pursued different policing strategies. And because such laws were not only introduced when crime was high, he doubts whether the effect was due to crime rates decreasing even if the add-ons had never been implemented.

Although infamous murder cases prompted politicians to pass some of these laws, the study did not include homicide. Because murder is considered a crime of passion — and sentences for homicide are longer than the typical gun enhancement — gun add-ons don’t provide much “bang for your buck” on homicide rates, Abrams said.

While economists like Abrams have found that sentence enhancements do have a deterrent effect, criminologists are typically more skeptical. In 2008, Michael Tonry of the University of Minnesota Law School published a critique of deterrence research in the journal Crime and Justice. Tonry pointed out that mandatory sentence enhancements are rarely applied faithfully by judges, who will often decrease an underlying prison sentence, like for robbery, if they are required to add additional years for the presence of a gun. He also called attention to the lack of research on whether would-be criminals have any knowledge about sentencing laws in the first place. In a widely-cited Criminology paper from 1995, Thomas Marvell and Carlisle Moody looked at crime rates across 44 states with gun add-ons and concluded, “We found little evidence to support the intended purposes of firearm sentencing enhancements, reducing crime rates and gun use.”

If the impact of gun add-ons is small or negligible, especially when it comes to preventing homicide, why do they remain on the books? Charles Decker, a doctoral student in political science at Yale University, is conducting new research on the politics of sentence enhancements. In an upcoming paper, he examines the Bartley-Fox act in Massachusetts, a 1974 law — the first of its kind nationwide — that required a mandatory minimum jail sentence of one year for possession of an unlicensed firearm outside of one’s home or place of business (even if no other crime was committed), and a two-year add-on for crimes committed while possessing a gun.

The bill was “a compromise measure,” Decker said. Spurred by rising crime, Massachusetts gun control advocates wanted to ban handguns, but pro-gun lobbyists blocked two bills that would have done so. The Bartley-Fox law was able to overcome opposition by appealing to both liberal and conservative critiques of the judiciary. Liberals believed judges were racially biased in sentencing, while conservatives attacked judges they saw as too liberal. Mandatory minimum sentences were a way to limit judicial discretion.

Bartley-Fox influenced the development of laws in states across the nation, as well as federal policy. In 1979, Senator Ted Kennedy introduced federal legislation modeled on Bartley-Fox. It would have required licenses for carrying guns, and violators would have faced a mandatory six-month prison sentence. “My proposal...seeks to find common ground between law-abiding, responsible handgun owners and those Americans who do not own or wish to own handguns,” Kennedy said.

The National Rifle Association opposed the legislation, arguing that only violent criminals with guns, not anyone who possessed an unlicensed firearm, should be subjected to mandatory incarceration. In the end, the NRA’s position held sway and became federal law — a law the current Senate proposal would modify, but not eliminate.

“This bill takes a 15 year term down to 10 years, which is still a really long time in prison,” said Molly Gill, the government affairs counsel at Families Against Mandatory Minimums. FAMM supports the legislation as a necessary but incomplete step forward. “Everyone is still going to prison under this bill. Everyone is still going for a really long time. That’s how modest this bill is.”