Cities make people more productive and innovative because physical proximity speeds the transmission of knowledge: workers learn from nearby workers, and skills and ideas pass more easily face to face than across distance. Glaeser treats this learning externality, not natural resources or climate, as the core reason urban workers earn and produce more.
Dense urban living produces lower per-household carbon emissions than suburban living because apartments share walls and require less heating, and short trips or transit replace long car commutes. Glaeser argues that environmentalism which blocks dense building in clean places like coastal California pushes development to higher-emission locations, worsening the footprint it intends to shrink.
A politician can entrench power by enacting policies that drive away opponents and attract loyal supporters, shrinking the city's tax base and prosperity while strengthening his own coalition. Named for Boston mayor James Michael Curley, the mechanism rewards a leader for making his jurisdiction poorer as long as it makes his remaining voters more dependent.
Policy aimed at reviving declining cities through buildings, stadiums, and infrastructure often fails because a struggling city's real asset is nothing but its people. Glaeser argues that investing in the residents' skills and mobility serves them better than pouring money into the physical shell of a place whose economic rationale has passed.
A city's poverty can be evidence of its success: places like Rio's favelas fill with poor migrants precisely because the city offers better prospects than the rural areas they left. Glaeser reframes the presence of poor people as a sign the city is doing its job of attracting those seeking upward mobility, rather than proof the city is broken.
Limiting how tall and dense buildings can rise, through preservation rules and height caps in cities like Paris and Mumbai, restricts supply and drives up prices, pushing residents outward. Glaeser argues that allowing more vertical construction where demand is high is what keeps a city affordable and accessible to newcomers.