Smith argued that trade among nations is like trade among people. No one feels compelled to sew his own clothes and grow his own food simply to keep busy. Instead, we find employment doing what we do best and rely on other people for most goods and services. Similarly, nations should specialize in producing what they do best and freely trade with other nations to satisfy their consumption needs.

This argument was expanded by David Ricardo in the 19th century. Ricardo addressed the question: What if one nation does everything better than another? His answer was that trade depends on comparative advantage — how good a nation is at producing one thing relative to how good it is at producing another.

Ricardo used England and Portugal as an example. Even if Portugal was better than England at producing both wine and cloth, if Portugal had a larger advantage in wine production, Portugal should export wine and import cloth. Both nations would end up better off.

The same principle applies to people. Given his athletic prowess, Roger Federer may be able to mow his lawn faster than anyone else. But that does not mean he should mow his own lawn. The advantage he has playing tennis is far greater than he has mowing lawns. So, according to Ricardo (and common sense), Mr. Federer should hire a lawn service and spend more time on the court.

By the way, Ricardo was not merely a theorist. He was also a successful stock trader and a member of Parliament. During his political career, he fought for free trade, notably, by opposing the Corn Laws, which imposed tariffs on grain imports.