In a classic study, the sociologist Eric Klinenberg showed just how important neighborhood is in determining who survives in a crisis. Klinenberg compared deaths in two Chicago neighborhoods during a heat wave in 1995. More than six times as many people died in North Lawndale as in South Lawndale, even though the two places are demographically comparable.

The fact is that human behavior happens in contagious, networked ways. Suicide, obesity and decreasing social mobility spread as contagions.

When you think in neighborhood terms rather than in individual terms you see things previously rendered invisible. For example, Klinenberg found that fewer people died in South Lawndale in great part because there was more social connection there. Klinenberg’s new book, “Palaces for the People,” emphasizes the importance of “social infrastructure,” physical places like libraries where people can gather. What do libraries have to do with deaths in a heat wave? It turns out quite a lot. Libraries nurture relationships among people who check in on one another when crises hit.