Before the financial crisis, the banking industry was too concentrated and clubby. And now? It’s even more so. In the midst of the crisis, the country’s four biggest banks—Citigroup, Bank of America, JPMorgan Chase, and Wells Fargo—actually got bigger. Thanks primarily to a series of government-sanctioned mergers, they now control almost forty per cent of the country’s total banking deposits and two-thirds of its credit cards, and issue half of all mortgages. Investment banking, too, remains more or less dominated by the usual suspects, which have seen their market share grow as the number of their competitors has shrunk. Firms that were recently on the brink of collapse haven’t had to struggle to hold on to their old customers, as...