As more people move to a city, you’d expect about a one-to-one increase in shirts being worn, for instance, or the number of house keys issued. If something doubles as population doubles, that’s not surprising. What is unusual, though, is when something grows faster or slower than a population. That means people seem to be doing more or less of it, on average, and that could signal an interesting societal quirk.
And that’s why a new paper out of Brazil's Universidade Federal do Ceara and the City University of New York is surprising. It shows that as cities get bigger, certain kinds of death become more common, and others become less so.
For the analysis, the authors looked at car crash fatalities, homicides, and suicides in all Brazilian cities between 1992 and 2009, as well as suicides in all U.S. counties between 2003 and 2007. The authors found that if they doubled the size of a Brazilian city, car-crash deaths would also double, as predicted. But the rate of murder would grow by 135 percent—that is, homicides would more than double.
The rate of suicides, meanwhile, increased slower than population growth, rising just 78 percent when population went up by 100. A similar trend was true among the U.S. counties. There seems to be something about big cities that makes murder more likely but suicide less so.