Inevitably, Romer’s big idea attracts some skeptical responses. “Paul is very creative,” says William Easterly, a development economist at New York University, “and sometimes creativity can cross the line into craziness.” The way Easterly sees it, charter cities (like charter schools in American cities) may provide an alternative to incumbent government systems, promising experimentation, competition, and perhaps a new way forward. But Easterly also worries that Romer has fallen prey to an old siren song—the idea that you can slough off debilitating customs and vested interests by constructing a technocratic petri dish uncontaminated by politics. Other critics are blunter. “Romer makes it sound as though setting up a charter city is like setting up a fairground,” Elliott Sclar, a professor of urban planning at Columbia University, told me. “We take a clear piece of land, we turn on the bright lights, and we create this separate environment that will stand apart from everything that’s around it. I wish it were that simple.”

However simple-seeming his ideas, Romer is no lightweight. Starting in the late 1980s, he produced a series of papers that changed the way his profession thinks about economic growth; his most celebrated contribution, published in 1990, “was one of the best papers in economics in 25 or 30 years,” in the estimation of Charles I. Jones, a colleague of Romer’s at Stanford. Before the Romer revolution, theorists had explained an economy’s growing output by looking at the obvious inputs—the number of hours worked, the skills of the workforce, the quantity of machinery and other physical capital.

But Romer stressed a fourth driver of growth, which he termed simply “ideas,” a category that encompassed everything from the formula for a new drug to the most efficient sequence for stitching 19 pieces of material into a sneaker. In statistical tests, the traditional inputs appeared to account for only half the differences in countries’ output per person, suggesting that ideas might account for the remaining half—and that leaving them out of a growth theory was like leaving the prince out of Hamlet. And whereas the old models had predicted that growth would slow as population expansion put stress on resources, and as new investment in skills and capital yielded diminishing returns, Romer’s New Growth Theory opened the window onto a sunnier worldview: a larger number of affluent people means more ideas, so prosperity and population expansion might cause growth to speed up.

Romer’s enthusiasm for technology made him a natural West Coaster, so it is not surprising that, after spells on the faculty at the University of Rochester and the University of Chicago, he fetched up at the University of California at Berkeley and then at Stanford’s Graduate School of Business. But the next turn in his thinking involved a rebellion against the libertarianism of his Silicon Valley home. “I was willing to be a bit confrontational,” Romer says, impishly. Starting with a paper he presented at a World Bank conference in 1992, Romer began to emphasize that “ideas” included more than just technologies and manufacturing processes. Ideas were also embodied in customs and institutions—or, as Romer later came to put it, “rules”—patent law, competition law, bankruptcy law, and so on, as well as the softer “norms” that govern people’s behavior. Indeed, these rules could be even more important than technologies, however much the digerati of Silicon Valley might wish to believe otherwise. Without new technologies, an economy might grow slowly. But without decent rules, an economy cannot even make use of the technologies that already exist.

To drive home the importance of good rules to economic growth, Romer sometimes shows a photograph of Guinean teenagers doing their homework under streetlights. The line of hunched, concentrating figures presents a mystery, Romer says; from the photo it is clear that the teens are not dirt poor, and youths like these generally own cell phones. Yet they evidently have no electric light at home, or they would not be studying by the curbside. “So here is the puzzle,” Romer declares: Why do these kids have access to a cutting-edge technology like the cell phone, but not to a 100-year-old technology for generating electric light in the home? The answer, in a word, is rules. Because of misguided price controls in the teenagers’ country, the local electricity utility has no incentive to connect their houses to the power grid. Their society lacks the rules that make technological advance meaningful.