The U.S. labor market is hot. Unemployment is at 3.8%, a level it’s hit only once since the 1960s, and many industries report deep labor shortages. Old theories of what’s wrong with the labor market — such as a lack of people with necessary skills — are dying fast. Earnings are beginning to pick up, and the Federal Reserve envisions a steady regimen of rate hikes.

So why does a large subset of workers continue to feel left behind?