Historically, trade systems have ranged from rather open to severely restricted by rules, tariffs, or non-tariff barriers, driven by shifts in the relative strength of liberalizing or protectionist economic and political forces. But even in closed systems, however severe the penalties they impose on trade, black markets usually develop, owing to the “gains from trade” generated by natural economic forces.
贸易开放的合理程度不是一个新辩题。从历史上看，贸易体系既有极其开放的，也有受规则、关税和非关税壁垒严格限制的，其变迁受自由派或保护主义经济和政治力量的相对优势驱动。但即使在封闭系统中，不管针对贸易的惩罚多么严格，黑市总是能够发展起来，这是拜经济力量自发产生的“贸易收益”所赐。