It might surprise you to learn that human cells commit suicide. The process is called apoptosis. What happens is, cells in a dense cluster send each other survival signals, but those that get isolated from the group begin to self-destruct. As the biologist Martin Raff once wrote, the only thing keeping cells from killing themselves "is that other cells are constantly stimulating them to live."
Turns out whole human beings may behave in a sadly similar way.
A new scientific working paper (spotted by Tim De Chant of Per Square Mile) contends that as population density decreases, the suicide rate among young people increases. This effect becomes particularly pronounced below 300 inhabitants per square kilometer — roughly the density of San Diego County. The research team, led by Chinese ecologist Lei Wang, wonders if social "shock" of moving from a dense city to a sparse countryside might have something to do with this unsettling link:
If young persons are more connected to their social environment, then the shock of moving out to a countryside environment will be more painful to them and, just as in the case of widowhood but with smaller magnitude, it will result in inflated suicide rates.
Let's backtrack a minute and consider density and mortality in general. The city has always seemed (at least to outsiders) like a hazardous place of crime and disease and countless interactions with complete and potentially dangerous strangers. But life at low densities has plenty of its own environmental perils: speedier roads and social disconnection, among them.
In fact, Wang and collaborators found no significant association between density and death on a broad scale:
When they narrowed their view down to young people ages 15 to 19, however, they found a clear statistical pattern showing that the death rate fell as population density increased (or vice versa). This was true for charts of all 50 American states plus D.C. as well as charts of about 1,000 U.S. counties. The strongest link held for external deaths, such as suicide and accidents; other types of death, such as death from disease, weren't really affected by density.
Here's a plot of the suicide rate from a smaller sampling of counties in states with low-density pockets (like Arizona and Utah) and states with high-density ones (like New York and New Jersey):
Other countries showed similar trajectories. In France and Canada, for instance, the connection between low densities and higher youth suicide rates seemed to hold. While the researchers didn't have statistics for young people in Japan, the density-suicide association appeared true for the general Japanese population. Germany didn't fit the model with any statistical significance, but the data did trend in the expected direction.
The big question, of course, is why low densities might have some relationship to high youth suicide and accident rates. One potential explanation is that living in a high-density city puts you closer to a hospital — but if that were the reason behind the connection, then people of all ages should show the same density-accidental death effect, and they don't. De Chant has looked at previous studies and found that greater access to weapons, and poorer access to mental health facilities, might also play a role in linking suicide and density.
In the end, since suicide of all causes of death relies most heavily on social networks, Wang and collaborators drew a comparison with apoptosis. Just as human cells that become isolated from others veer toward death, human beings who live in areas below 300 people per square kilometer may have a greater chance of losing healthy social support. Still the researchers admit that they don't have all the answers; their purpose was "not to explain everything," they write, but rather to describe a statistical pattern and propose one theory worthy of larger discussion.
As De Chant points out, city people can find both good and bad sides to the research: good in that very dense areas don't (on their own) tempt us toward suicide, and bad in that they don't (at least above 300 people per square km) inoculate us further from it. But there are lessons here for low-density places, too. When you escape the city to leave its hazards behind, you also say goodbye to many of its benefits.