After three decades of progress, upticks in violent crime rates across many U.S. cities in the past two years have created a popular perception that crime is out of control, even as most Americans have not felt this bump. Violent crime remains concentrated in places within cities that never benefited from the great crime decline of the 1990s and 2000s, places where violence has long been entrenched and higher than surrounding areas. The concentration of violence is just as important as the overall rate of violence, as it has long-term effects on health and economic mobility. A dual strategy of focused policing and neighborhood investment could improve violent crime rates in places where violence pools. But first, we must properly measure concentration and understand its causes.
Crimes of all types cluster in a small share of individual locations, often called microgeographic units. In a foundational 1989 study of Minneapolis, Larry Sherman and colleagues reported that 50 percent of calls to police in a single year came from just 3.3 percent of street addresses and intersections. David Weisburd and colleagues found a similar concentration of crime among street segments in Seattle over 14 years. Dozens of studies conducted in the past decade have confirmed these findings. Though the upper and lower estimates vary, the consistent conclusion that a large portion of crime is perpetrated in a small set of locations has yielded the “law of crime concentration.” What’s more, crime concentration is remarkably stable over time.
But how researchers report crime concentration to policymakers matters. As it turns out, the traditional framework of X percent of crimes in Y percent of places is not very helpful. After all, major cities contain tens of thousands of microgeographic units. Similarly, knowing that crime is concentrated in a few areas is not the same as knowing how likely a particular place is to experience a crime. Misstating the concentration of crime can lead to strategies that put too many police resources in the wrong places.
When crimes are scarce but costly, as with murder, policymakers need a relative metric that tells them if the concentration is indeed aberrant. A better option for explaining crime concentration is a measure of statistical dispersion akin to the Gini coefficient, which avoids the pitfalls of overestimation but is sometimes difficult to interpret. Still, better might be a measure of marginal concentration proposed by Aaron Chalfin and colleagues. They estimate the difference between the actual distribution of crime and the distribution expected if each crime were equally likely to occur anywhere in a city. This method is also resistant to overestimation and provides policymakers with a single, accurate metric that is easy to translate into decisions about resource allocation.
Simply knowing that crimes occur in a small share of locations isn’t enough, though. Because policymakers must make tradeoffs, it is critical to help them understand why crime is spatially concentrated. Weisburd and colleagues found two particularly potent explanations in Seattle that seem consistent across the rest of the country. First, areas that lack social organization – that is, those communities where deprivation and segregation are exacerbated by poor cohesion among neighbors – tend to host high concentrations of crime. Such neighborhoods are characterized by physical disorder and poverty. They also tend to have low property values and high school truancy rates. Second, within these areas, there are often specific locations where perpetrators are likely to encounter their victims with impunity. These causes are correlated, but they have different policy implications.
Policymakers have several levers by which they can reduce the spatial concentration of crime without spreading the same amount of crime over a larger geographic area – an important goal in communities where economic inequality is compounded by endemic violence. Social interventions affect whole neighborhoods rather than microgeographic units, and they are relatively expensive and slow to pay off — but their effects last. Summer jobs programs for youth seem particularly apt to reduce violent crime, as do investments in mental health access. But what explains crime at the neighborhood level doesn’t always translate into how to solve crime concentration at specific places within that neighborhood.
Characteristics of microgeographic areas have long been understood to attract concentrated crime, particularly opportunistic crimes such as robbery or assault. Enforcement interventions in specific places can be cost-effective and have immediate payoffs without causing crime to spill over into other areas, but their effects dwindle over time. For instance, gang takedowns, a form of targeted enforcement, are shown to significantly reduce violent crime in public housing projects, but only for about a year and a half after they’re implemented. Changes to the physical environment, such as lead abatement and the installation of supplemental lighting, offer benefits at both the neighborhood and microgeographic levels without the need for increased policing.
In practice, because neighborhood and place-based factors interact to exacerbate crime concentration, a mixed strategy is required. Social investment, targeted enforcement, and changes to physical environments within neighborhoods combine to yield less crime concentration and less total crime. To determine the right mixture, policymakers must first accurately characterize the concentration of crime in their jurisdiction and triage resources to address the crimes and places that generate the most harm. All this takes finesse and can’t be accomplished through measuring crime concentration alone.
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