The Panama Papers—the massive collection of leaked documents from Mossack Fonseca, a Panamanian law firm that helps set up offshore shell corporations—have already had political consequences. Iceland’s prime minister, Sigmundur David Gunnlaugsson, resigned after the leak revealed that he had partly owned an offshore firm. David Cameron, the British prime minister, is facing criticism over an offshore company that his father set up. In Brazil, many of the people connected to the country’s unfolding corruption scandal appear to have held offshore shell companies set up by Mossack Fonseca. And in Russia, Sergei Roldugin, a cellist who