1. Get the lead out.

A lot of theories have been posed for explaining the fall in crime rates, and in particular violent crime rates, since the 1990s. Did "broken windows" policing work? What about data-driven approaches like CompStat? Was it that legal abortion allowed unprepared mothers to delay parenthood, and spare their kids childhood environments that might encourage criminality later on? Or does the falling price of cocaine explain it?

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None of the above. The real answer, it's now becoming clear, is lead. In the 1970s, the environmental movement succeeded in getting lead out of gasoline and household paint, and the result has been smarter, less violent kids. Economist Rick Nevin has found that, if you add a 23-year lag, variations in lead exposure explain 90 percent of the variation in crime rates in the United States.

Jessica Wolpaw Reyes, an economist at the Amherst College, found that declining lead exposure caused a 56 percent decline in crime from 1992 to 2002, a decline that was reversed by other factors to leave the actual decline at 34 percent over that period. Wolpe Reyes has also found significant effects on childhood delinquency and academic performance. The correlations are simply staggering. Kevin Drum laid out the evidence in a long piece earlier this year, including this striking graph.

Neurological research has indicated how this effect works. Lead hurts the parts of the brain responsible for executive functioning (emotional control, control of impulses, etc.), and aggression control, and makes it harder for neurons to communicate with each other. It's no wonder it leads to all kinds of social maladies.

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Drum suggests that a $20 billion a year investment in deleading, over the course of a few decades, would generate $30 billion a year in new income by increasing children's IQs going forward. He guesses that a 10 percent decline in crime is a reasonable outcome to expect.

2. Double, triple, quadruple the alcohol tax.

As criminologist Mark Kleiman told me last month, "Any sentence about drug policy that doesn’t end with 'raise alcohol taxes' is an incoherent sentence." He's hardly the only one with that view. Economics, criminology and public health literature are rife with studies finding that raising the price of alcohol reduces violence, not to mention other causes of injury and death. Indeed, every self-reported survey of incarcerated criminals suggests that 36.8 percent of state-level violent offenders, and 20.8 percent of federal violent offenders, were drinking when they committed the crime for which they're incarcerated.

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Economist Sara Markowitz, for example, found in a study of U.S. crime patterns that a "single percent increase in the beer tax decreases the probability of assault by 0.45 percent" and "a 1 percent decrease in the number of outlets that sell alcohol decreases the probability of rape by 1.75 percent." Researchers in Finland found that a 2004 cut in the country's alcohol tax caused a sudden 17 percent spike in fatalities relative to the previous year. There's preliminary evidence that alcohol taxes can reduce the number of U.S. female homicide victims. Kleiman cites findings of Duke's Philip Cook to the effect that a doubling of the federal excise tax on alcohol would reduce homicide and automobile fatalities by 7 percent each, for a net 3,000 lives saved. What's more, it would only cost twice-a-day drinkers (who, as it is, drink considerably more than average) $6 a month.

One common doubt surrounding this method of reducing violent crime is whether or not alcoholics really care what the price of alcohol is. If they don't, then raising the price would just cost them money without much social benefit. But the data suggests that alcohol consumers are, in fact, sensitive to the price of the product. Researchers at the CDC compiled a number of studies estimating "price elasticities" of different kinds of alcohol.

The elasticity (as Stringer Bell explains above) is the amount by which consumption changes when the price of a good changes. For example, if the elasticity is -0.5, then a 10 percent increase in price will reduce consumption by 5 percent. If the elasticity is 0 or positive, then consumers are either indifferent to the cost of the product or actually want it more because it's more expensive.

A few studies found that the elasticity of spirits is positive, perhaps because expensive Scotch or bourbon are Veblen goods: people buy them more when they're more expensive because their expense makes them better status markers. But generally, the elasticities are negative. Raising the price of alcohol, for example by raising the tax on it, is an effective way to reduce consumption, and thus alcohol-related fatalities and assaults:

Researchers Alexander Wagenaar, Amy Tobler and Kelli Komro also conducted a literature review on alcohol tax and price policies, scanning through 50 studies on the subject. Their conclusion: "Our results suggest that doubling the alcohol tax would reduce alcohol-related mortality by an average of 35%, traffic crash deaths by 11%, sexually transmitted disease by 6%, violence by 2%, and crime by 1.4%." The case for making higher alcohol prices a part of our approach to reducing violent crime, then, is pretty strong.

3. Foster care for young delinquents.

The one crime policy which the Coalition for Evidence-Based Policy rates as "top tier" — meaning it's been repeatedly shown to work in randomized controlled trials — is a program known as "Multidimensional Treatment Foster Care." In MTFC, severely delinquent children are removed from their homes for a period of six to seven months and placed with foster families. Under usual policy, such children tend to be placed in group homes with other delinquent youth, rather than in a family setting.

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The foster parents are generally members of the child's community and are given special behavioral training to deal with troubled youth. The children receive both individual and group therapy (of the cognitive-behavioral variety which has itself been shown to work in randomized trials as well). Special care is taken to make sure the children aren't in contact with other troubled kids.

Three randomized studies have evaluated MTFC and all have found significant positive effects. A study on delinquent girls, average age 15, found that those in foster care, rather than a group home, spent 69 percent fewer days in jail or correctional facilities over the next two years, had 55 percent fewer criminal referrals, and had a 38 percent lower pregnancy rate. Another study found that boys in the program had 45 percent fewer criminal referrals and 68 percent fewer self-reported violent incidents. The third found strong effects on teen pregnancy, and has yet to follow-up on other measures.

4. Better police tactics

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Yeah, "broken windows" probably didn't deserve the hype. But police really can reduce the crime rate. In their aptly named literature review, "What Can Police Do to Reduce Crime, Disorder, and Fear?" the criminologists David Weisburd and John Eck point to "hot spots" policing, in which police identify particular geographic locations with high crime and then focus efforts there, as a proven tactic. As I explained earlier this year, they found five randomized studies supporting both "hot spots" and a closely related method, "problem-based policing," as strategies.

The Minnesota Hot Spots Patrol Experiment, conducted from June 1987 to June 1988, found that a hot spots strategy resulted in a 6-13 percent reduction in total crime reports. The Kansas City Crack House Raids Experiment, undertaken from November 1991 to May 1992, found an 8 percent net reduction in crime, though those results decayed as the strategy continued. A study in San Diego found that a kind of hot spots policing in which police raids are followed up with visits to landlords to make sure activity has not started up again results in a 60 percent reduction in crime, relative to having no follow-ups with landlords.

But the most interesting studies they highlight focus on a “problem-based” policing, which tailors police methods to particular problems (like drug dealing, or gang violence) and tries to incorporate other government services in the process. Both randomized experiments in this area focused on Jersey City, N.J. One found that hot spot policing combined both with target-specific tactics (videotape surveillance of public spaces, confiscating guns stashed in public areas) and with social service intervention (including aid to the homeless, street trash removal, and better enforcement of liquor and housing codes) resulted in significant crime reduction. The second compared a normal hot spot intervention to one that incorporated local regulatory agencies and “problem-oriented” tactics, and found the latter to be more effective.

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Of course, all these techniques require an increase in the police force, which may be costly and hard to push through politically.

5. After-school sports.

The University of Chicago’s Crime Lab evaluated a program called “BAM — Sports Edition,” which provides 7th-10th-grade boys with small group instruction in social and life skills in school, and sports programming after school. The study, which used a randomized design, found that the program reduced violent crime arrests by 44 percent. The authors estimate that the program's social benefit probably total "$49,000 to $119,000 per participant from increased lifetime earnings, tax payments, and lower public benefit use."

6. Preschool.

Yep, that too. There's some reason to believe that high-quality preschool programs reduce crime rates among their participants. Of the two marquee experiments on the topic, the Perry project found significant reductions in crime by age 40, while the Abecedarian project did not find any effects. Then again, the last follow-ups in Abecedarian were conducted when participants were only 21, so it’s possible that a gap opened up later on.

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So this area is less conclusive than others. But the Perry intervention, at the very least, had a real effect on crime rates, which suggests that replicating such an impact in another preschool program is at least possible. As Nobel laureate and preschool advocate James Heckman has written, "In evaluating drugs to control blood pressure, we do not dwell on the failures except to learn from them. We should implement the successes."

7. Target gangs.

In a paper coauthored with Duke’s Phil Cook, the University of Chicago Crime Lab’s Jens Ludwig suggests that “gang-based deterrence,” in which gangs are collectively held responsible by police and other government and civil society respondents for violence committed by their members, might be more effective than targeting individual perpetrators.

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The Kennedy School’s Anthony Braga, David Kennedy, Anne Piehl and Lehman College’s Elin Waring found that the Boston Ceasefire program, an intervention in the mid-1990s that used a gang-based approach, was associated with reductions in violence, though their evidence is non-experimental and cannot determine what specific parts of the ceasefire program caused the reductions.

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8. Take away already illegal guns.