Integration is the best way to make our cities safer.

In my last blog post, I wrote about how much we may be able to learn about effective crime prevention by comparing experiences across cities. The following week, I had a fascinating exchange with Richard Florida about which elements of city life are most important in preventing crime.

Our discussion prompted me to take a step back and ask: “What is the most important element of a place with respect to crime and public safety?” To answer, I’d like to conduct a thought experiment showing that economic segregation leads to the worst crime outcomes and that the best way to reduce crime is to find ways for the rich and poor to live next to each other. This is not based on some utopian fantasy, but rather on logic.

Crime is contagious. Of that, there is little doubt. A central predictor of crime levels in a neighborhood is how much crime there is in the neighborhood next door. Communities that share borders share crime (technically, this is known as spatial autocorrelation).

Over the past few decades, criminologists have become convinced that places can make people more crime-prone than they otherwise would be. Fix the place, and the people will be safer. For instance, a place with lots of alcohol outlets will almost certainly have more violence than an otherwise identical place with few places to buy liquor.

The best analogy is the spread of a virus. Each individual exposed to a virus has some chance of getting sick, but those with weaker immune systems have a greater risk of falling ill. Places bear similar risks. Certain features—poverty, density, isolation—put some places at greater risk of more crime and violence.

Here is the thought experiment: Suppose we set up the worst-case scenario, one where cities have no recourse to reduce crime other than arranging where people live. And suppose a city was assigned a set number of high-risk (economically disadvantaged) places, a set number of low-risk (highly prosperous) places, and some in between. Now we ask, how should the city arrange those places to create the most safety and the least amount of crime?

The answer is surprising. While we know that isolating our poorest residents is really bad for them, it turns out that segregating the rich and poor leads to the worst outcomes for a city as a whole. Economic integration, where the rich and poor live side by side, leads to the safest cities.

To test this hypothesis, I have borrowed some Brookings Institution models describing the spread of viruses. Working with our D.C. Crime Policy Institute, the Brookings scholars determined that at least 70 percent of crime in Washington, D.C., is due to contagion (the spreading of crime like an epidemic).

How does that work? Below, I describe a city as a 10 X 10 grid. Each of the 100 cells in the grid is a neighborhood. The neighborhoods have some intrinsic ability to fight a crime infection. The dark shaded areas are the most isolated, poorest, densest neighborhoods—and for this very stylized model, I assume they are *all* infected with the crime bug. The in-between areas (the lightly shaded areas) have a 25 percent chance of becoming infected if they are "exposed" to a neighboring community with high crime. The prosperous neighborhoods (the unshaded areas) are far more resilient; they only have a 5 percent chance of being infected if exposed.

First, let’s consider a perfectly segregated city. All the high-crime neighborhoods are in the same part of the city. All the prosperous places are physically isolated from the high-crime places by the in-between neighborhoods.

Now, let’s explore how an epidemic spreads. Below, we look at a section of our city where the infected places interact with their healthier neighbors. Infected neighborhoods are identified with a number, the in-between neighborhoods with a letter, and the prosperous areas are blank.

I assume there is a one-in-four chance that a high-crime neighborhood will infect its next-door neighbor, and thus every fourth "in-between" neighborhood touching an infected neighborhood becomes infected itself. Place 1 touches place A and place B. Place 2 also touches place A and B, so place B is the fourth neighborhood touched, meaning it becomes infected. Following this pattern, the bold letters below indicate the neighborhoods that become infected.

Place 1—A, B

Place 2—A, **B**, C

Place 3—B, C, **D**

Place 4—C, D, E

Place 5—**D**, E, F, G, **H. **(Since place D is already infected, we’ll just mark place E as infected in the grid below to properly show the crime bug’s reach).

So, on our first pass, we see four new places infected with crime.

Now, places B, D, E, and H are sick, with a 25 percent chance of infecting their neighboring communities, and so on.

When you work through the entire epidemic, you find that the 25 infected neighborhoods infect an additional 17 neighborhoods. By segregating and isolating the most at-risk places, crime is not corralled, but rather spreads more effectively.

Now imagine that instead of surrounding the infected places with in-between places, we instead surround the infected places with the most prosperous places. Since only 1 in 20 (5 percent) of prosperous places is infected when exposed to the crime virus, then only one prosperous neighborhood is infected in this thought experiment.

Thus, having rich and poor live side by side means 26 of our 100 neighborhoods have crime problems. That is a vast improvement to absolute segregation, where 42 of our 100 neighborhoods are infected.

Of course, this thought experiment ignores the positive benefits of vaccinating places. Cities have many potential vaccines in their arsenal to prevent the spread of a crime epidemic. Identifying which vaccines are most effective is at the core of the discussion Florida and I began about which elements of city life are most important in preventing crime. But it is helpful to begin that discussion by framing it in terms of contagion, epidemics, and vaccines.

The bottom line is, just as we cannot arrest our way out of crime problems, we also cannot economically segregate and isolate our way out either. That approach is self-destructive and has led to many of the problems our cities face today. Figuring out how to fix those mistakes is at the core of creating prosperous places.

*Top image: Uros Zunic/Shutterstock*

*This post originally appeared on the Urban Institute's Metrotrends blog, an Atlantic partner site.*