All economic data are best viewed as a peculiarly boring genre of science fiction, but Chinese data are even more fictional than most. Add a secretive government, a controlled press, and the sheer size of the country, and it’s harder to figure out what’s really happening in China than it is in any other major economy.

Yet the signs are now unmistakable: China is in big trouble. We’re not talking about some minor setback along the way, but something more fundamental. The country’s whole way of doing business, the economic system that has driven three decades of incredible growth, has reached its limits. You could say that the Chinese model is about to hit its Great Wall, and the only question now is just how bad the crash will be.

Start with the data, unreliable as they may be. What immediately jumps out at you when you compare China with almost any other economy, aside from its rapid growth, is the lopsided balance between consumption and investment. All successful economies devote part of their current income to investment rather than consumption, so as to expand their future ability to consume. China, however, seems to invest only to expand its future ability to invest even more. America, admittedly on the high side, devotes 70 percent of its gross domestic product to consumption; for China, the number is only half that high, while almost half of G.D.P. is invested.

How is that even possible? What keeps consumption so low, and how have the Chinese been able to invest so much without (until now) running into sharply diminishing returns? The answers are the subject of intense controversy. The story that makes the most sense to me, however, rests on an old insight by the economist W. Arthur Lewis, who argued that countries in the early stages of economic development typically have a small modern sector alongside a large traditional sector containing huge amounts of “surplus labor” — underemployed peasants making at best a marginal contribution to overall economic output.