The U.S. economy right now feels a bit like a bar stool that has lost a few of its legs. As long as the remaining ones hold, everything looks stable enough. But the fewer supports you have, the more vulnerable the whole thing becomes. That is increasingly how the current economic expansion looks. The headlines still show growth, unemployment is relatively low, and consumer spending has not collapsed. Yet beneath those numbers, the economy appears to be relying on a surprisingly small number...