How golden ages really start—and end

The way to start a “golden age” is to erect big, beautiful barriers to keep out foreign goods and people. That, at least, is the view of the most powerful man on the planet. Johan Norberg, a Swedish historian, makes the opposite case. In “Peak Human”, Mr Norberg charts the rise and fall of golden ages around the world over the past three millennia, ranging from Athens to the Anglosphere via the Abbasid caliphate. He finds that the polities that outshone their peers did so because they were more open: to trade, to strangers and to ideas that discomfited the mighty. When they closed up again, they lost their shine.