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Peak Human. By Johan Norberg. Atlantic Books; 400 pages; $32.99 and £22

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.

Consider the Song dynasty in China, which lasted from 960 to 1279AD. Song emperors were much keener on the rule of law than their predecessors, who tended to rule by whim. To enforce predictable rules, they hired lots of officials via meritocratic exams. The first Song emperor enacted the “unconventional policy reform” of “[not] killing officials who disagreed with him”.

Peasants were granted property rights and allowed to move around, rather than being tied to a lord’s land. Farm output more than doubled, and the extra food supported much larger cities. In the 1100s Kaifeng, the capital, had 65 times the population of London. Canals made domestic trade easier. International trade followed. Merchants started issuing paper money, six centuries before Europeans did, and the government embraced this brilliant idea—so much easier than carrying heavy strings of copper coins.

“Crowded cities set the stage for an unparalleled exchange of ideas, goods [and] services,” notes Mr Norberg. Artisans devised new industrial processes, such as burning coal to smelt iron. The invention of movable type in the 1040s allowed the printing of books so cheap that one philosopher griped that people would stop learning the classics by heart. By 1200 Song China had the world’s richest economy, a merchant navy with “the potential to discover the world” and a habit of tinkering that could have brought on an industrial revolution centuries before Europe’s. But then the Mongols arrived.

How golden ages really start—and end