In his 2000 book Bowling Alone, the Harvard sociologist Robert Putnam popularized the concept of “social capital” – the community bonds that underlie civic life. Putnam famously claimed that organizations such as bowling leagues not only provide a pleasant social outlet, but also form the basic substructure for society itself. Community and social groups, from bowling leagues to churches, are therefore vital for the future of American civil life. But social capital has a dark side, too – one that progressives and cosmopolitans have long recognized. Tight social bonds can isolate groups from one another, solidify class and gender inequalities, and exclude “undesirables” from valuable social networks. And now, research in Germany has found perhaps the most chilling side effect of social capital yet: it helped the spread of Nazism.