THREE DECADES ago, a German history professor listed 210 proposed explanations for the fall of the Roman Empire. The remarkable array included such fanciful causes as Bolshevism, public baths, hedonism, the pressure of terrorism and, most famously, lead poisoning. The last explanation has been discredited. It is highly unlikely that lead water pipes caused the ...

American political commentators have frequently called for the U.S. president to take effective action to improve world economic growth. Such calls are a form of what Matthew Yglesias has dubbed “Green Lanternism”—the unspoken theory that the U.S. president’s ability to affect outcomes is primarily affected by his willpower. In this article, I examine the opposite—and ...