In the era of the iPhone, Face­book, and Twit­ter, we’ve become enam­ored of ideas that spread as effort­less­ly as ether. We want fric­tion­less, “turnkey” solu­tions to the major dif­fi­cul­ties of the world—hunger, dis­ease, pover­ty. We pre­fer instruc­tion­al videos to teach­ers, drones to troops, incen­tives to insti­tu­tions. Peo­ple and insti­tu­tions can feel messy and anachro­nis­tic. They intro­duce, as the engi­neers put it, uncon­trolled variability.