Border effects and the availability of domestic products abroad
研究国家间贸易低于国内贸易的原因，区分了集约边际（同产品贸易量少）和扩展边际（部分产品不出口），发现两者各解释约一半的边界效应。
Abstract. Trade between countries could fall short of trade within a country because (1) the volume of international trade is less than the volume of domestic trade for a given product (the intensive margin); or (2) some goods that are sold domestically are simply not exported (the extensive margin). My theoretical model illustrates that either of these two factors could explain a given aggregate ‘border effect.’ I examine the empirical relevance of this distinction by isolating the fraction of total domestic production attributable only to exporters, finding that around one‐half of the border effect may be attributed to each explanation.