To the researchers, this suggests that the decision to commit murder or suicide, “instead of being purely a consequence of individual choices, might have strong correlations with the underlying complex social organization and interactions.” In other words, suicide and murder aren't random; they're products of our life circumstances.

ArXiv.org

In some ways, the suicide part makes very little sense. Cities are stressful; they’re polluted, crowded, and expensive. Metropolises like New York have the ability to make people feel completely alone while surrounded by millions of people. And yet: There’s something about that dense coexistence that seems to work like a buffer against the deepest of depressions.

Social interaction makes us happier, so it's possible that big cities simply provide more opportunities to interact and share experiences. As the authors put it, “a large supply of potential social contacts and interactions might work as an ‘antidote’ for this tragic event. This result is consistent with the idea that human happiness is more a collective phenomenon than a consequence of individual well-being conditions.”

Most suicidal people want to live, but they see no other way to escape their problems. My other theory for this phenomenon is that populous areas offer more options across the board—in access to mental-health practitioners, types of jobs, and/or the ability to find a new mate or social group—so fewer people feel like they have no other choice but to end it all.

Brazil in general has a very low suicide rate and high murder rate, but the fact that suicide rates in big American cities were similarly low seems to corroborate the Brazil data. The most suicidal U.S. cities are the relatively mid-sized and sparsely populated towns of Las Vegas, Colorado Springs, and Tucson.

If anything, this study is a bit of a double-edged sword for advocates of population density. Living in a teeming mass makes you less likely to experience inner turmoil, it seems, but more likely to want to off your neighbors. Hell might be other people, but they might just save you from yourself.