By the end of the 1920s three big U.S. companies dominated the car industry: General Motors, Ford and Chrysler. All of them were based in Detroit, which soon was nicknamed Motor City. By 1950, the U.S. produced three quarters of all cars in the world. And as vehicles became ubiquitous, gasoline consumption increased, pollution ensued…and violent crimes rose. In the 1950s, violent crime increased sharply and continued doing so until the early 1990s when it reached its peak. It was not until 1996 that scientists won their cause and crime rates began to decrease the following years: lead, a highly poisonous chemical, was prohibited as an additive in gasoline. But, ¿how is lead related to violent crime?

“Lead alters the formation of the brain especially affecting aggression and impulse control”, said Federico Curci, PhD candidate at the University of Carlos III Madrid.  “Medical literature has proven that lead is especially harmful to children ranging from the age of 0 to 2 as the brain absorbs it thinking it’s calcium”, he adds. Lead exposure increased violent attitudes in children and crime rates peaked as they grew up to become young adults. Curci presented on January 15th his job market paper Flight from Urban Blight: Lead Poisoning, Crime and Suburbanization at the University of Navarra, which investigates the effect of this chemical on crime and the urban structure of U.S. cities.

In the 50s and 60s lead use in gasoline increased exponentially. It wasn’t until 1965 that the first study arose about the medical effects of this chemical. In the next decade the fight towards its illegalization reduced its use, but humans were already exposed to it through many ways, but mainly soil. “You could be exposed through direct contact at a playground, with air resuspension or even with the dust that you brought home with your shoes”, commented Curci.

This situation, however, didn’t affect everyone equally. In his research, Curci differentiates between “good” and “bad” soil, the former being a near-neutral soil, one between 6.5 and 7.7 ph. “Good soil retains lead and doesn’t spread it to human beings, reducing its harmful effect”, he explained.  The difference in soil amongst U.S. cities would explain why each city was affected differently by this common trend.

The author then uses soil differences and the national level of lead poisoning across years to obtain an exogenous, and perhaps more interestingly, biological component of violent crimes. His results suggest that as crime rises people decide to move to the outskirts of the city, resulting in a suburbanization process. The latter has aggregate effects on the city.

Detroit is, perhaps, the best example of a U.S. city in which this happened. People escaped from an increasingly dangerous city center and they were followed by manufacturing jobs. However, the Motor City was only one of many cities that experienced this situation. Despite increasing urbanization, with a record 66% of the world’s population estimated to be living in a city by 2050, less people are now choosing to live in the city center. For example, from 1960 until 1990, Philadelphia’s suburb population increased from 2.4 million to 3.5 million, but it’s city center decreased from 2 million to 1.6 million. “The overall increase in crime has implied a 35% decrease in population of city centers, meaning that more than 25 million people have moved to the suburbs”, argued Curci.

After lead was illegalized in 1996, crime rates lowered in U.S. cities, but its urban structure didn’t go back to their previous configuration. “People didn’t move back to the city center as there had been lots of money invested in the suburbs: school districts had been created, real estate institutions had evolved and a wide variety of amenities were established in the outskirts of the city”, Curci remarked.