HOW PRODUCTIVITY INCREASES:

Liberalization has generally served us well. Trade has delivered geopolitical benefits in many, though not all, cases. After World War II, Europe traded together and avoided another war. Or consider Mexico. Since joining NAFTA (the North American Free Trade Agreement) in 1994, its exports to the United States have expanded more than sevenfold. (Over the same period, U.S. exports to Mexico rose fivefold.) This has helped stabilize Mexico's economy: something in U.S. interests.

Of course, there are costs, most obviously job loss. One study published by the National Bureau of Economic Research estimates that Chinese imports eliminated between 2 million and 2.4 million U.S. factory jobs from 1999 to 2011. That's roughly 40 percent of the 5.6 million lost manufacturing jobs in these years. Not everyone agrees. Harvard economist Robert Lawrence says continuous production efficiencies explain most job loss.

Whoever's right, there's a larger truth: Swings in the U.S. labor market, now with 148 million jobs, depend mostly on the domestic economy's health. Meanwhile, imports provide other economic benefits: lower prices, more consumer choice.

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