Trade within versus between nations: ...economics does not offer unconditional policy prescriptions. Every graduate student learns that depending on the background specifications, any policy x can be good or bad. A minimum wage can lower or raise employment (depending on whether employers have monopsony power); a natural resource discovery can raise or lower growth (depending on the likelihood of the Dutch disease); fiscal consolidation can expand or contract output (depending on the respective strengths of expectational versus Keynesian effects). And yes, the dictum that free trade benefits a nation depends on a long list of qualifying conditions.
So the proper response to the question “is free trade good?” is, as always, “it depends.” When an economist says “I support free trade” s/he must mean that s/he judges the circumstances under which free trade would not be desirable to be very rare or unlikely to obtain in the context at hand.
Many of the conditions under which free trade between nations is guaranteed to be desirable are unlikely to hold in practice. Market imperfections, returns to scale, macro imbalances, absence of first-best policy instruments are ubiquitous in the real world, particularly in the developing world on which I spend most of my time. This does not guarantee that import restrictions will be necessarily desirable. There are many ways in which governments can screw up, even when they mean well. But it does mean that a knee-jerk free trader response is faith-based rather than science-based. ...
[He goes on to answer a question about differential support for trade within nations versus trade between nations.]