Big cities breed partners in crime

Researchers have long known that bigger cities disproportionately generate more crime. Now a new study from Northwestern University and the Santa Fe Institute explains why: It's easier for criminals to find collaborators.

"In a big city, you have the potential to meet more distinct people each day," said Northwestern's Daniel Abrams, senior author of the study. "You're more likely to find an appropriate partner to start a business or invent something. But perhaps you're also more likely to find the partner you need to commit a burglary."

The study was published this week (Sept. 17) in the journal Physical Review E.

Abrams is an associate professor of engineering sciences and applied mathematics in Northwestern's McCormick School of Engineering. Vicky Chuqiao Yang, a postdoctoral fellow at the Santa Fe Institute and a former Ph.D. student in Abrams' lab, is the paper's first author.

As cities grow, crime grows even faster. And certain types of crime -- such as robbery, car theft and murder -- exponentially outpace the population.

"If you double the size of a city, you don't just double the amount of crime," Abrams said. "You actually more than double it."

To learn why, Abrams and Yang collaborated with Northwestern sociology professor Andrew Papachristos, who studies social networks in urban neighborhoods. The researchers developed a new mathematical model that predicted the number of crimes as a function of social interactions.

To do this, they used data from the FBI, Chicago Police Department and the National Incident-Based Reporting System. The team was specifically interested in co-arrest records -- or records in which multiple people were arrested for the same crime -- across seven categories: robbery, motor vehicle theft, murder, aggravated assault, burglary, larceny-theft and rape. Of these categories, only rape grew linearly, at roughly the same pace as a city's population. The other crimes are more "social" in nature and often require a team effort.

Although this project focuses on crime data, the researchers stress that their model also applies to positive outcomes. For example, big cities also produce more patents, more small businesses and more income per resident.

"The world is becoming very quickly urbanized," Abrams said. "There is a huge mass migration to cities from rural areas. It's important to understand what good and bad effects come with that. In order to promote the good side and reduce crime, we have to understand why it occurs."