Optimists are counting on the growing Chinese consumer class to spend the world back to prosperity. But a new study suggests that won't be so easy.

If, as I argued in my previous column, the free-spending, never-met-a-credit-card-they-didn't-like U.S. consumers aren't coming back within the next decade, who will pick up the slack? Who will buy all the cars, the flat-screen TVs and the steel that the global economy is geared up to produce?

The hopeful answer is China and its growing numbers of middle-class consumers. Not only does that country's growing wealth add hundreds of millions of buyers to the markets for products as varied as designer sunglasses and air conditioners, but the Chinese government is also committed to policies that will grow domestic...