The “New Trade Theory” -monopolistic competition
In contrast to the classical theories, the “New Trade Theory” (Krugman, 1979; 1980) explains why countries engage in intra-industry trade. This is a valuable result because the great bulk of global trade is intra-industry rather than inter-industry in nature. The ability of the theory to explain this feature of global trade is made possible by a number of assumptions: consumers prefer variety in consumption, the market is populated by firms selling different varieties of a good and there are increasing (internal) returns to scale in production, meaning that a firm’s average cost of production falls as its volume of production increases.
The theory predicts that trade costs can have a disproportionately adverse impact on small developing economies. Typically, small developing economies have large agricultural or natural resource sectors typified by constant returns to scale, and only a small manufacturing sector. In contrast, big developed economies have a large manufacturing sector operating under increasing returns to scale. In this setting, trade costs lead both to less trade and to a disproportionate relocation of manufacturing to the big developed countries (the “home market effect”). Meanwhile, small developing countries become concentrated in the agricultural or natural resource sector.
The key to explaining this result lies in the tension created between the consumer’s love of variety and increasing returns to scale. With open trade and zero trade costs, consumers in the big developed country will purchase both foreign and domestic manufactured goods because of their preference for variety. All things being equal, love of variety leads to more trade. On the other hand, increasing returns to scale gives a cost advantage to manufacturing firms in the developed country because of the size of the market and the larger scale of production that could be achieved by firms there. All things being the same, consumers in the developed country will prefer to purchase lower-cost domestic varieties than higher-cost foreign varieties.
无“新贸易理论”——垄断竞争
和古典理论相反，“新贸易理论”（克鲁格曼，1979；1980）解释了为什么各国从事产业内贸易。这是一个很有价值的成果，因为大部分全球贸易是产业内贸易而不是产业间贸易。这一理论通过一系列假设实现了对全球贸易这一特征的解释：消费者更喜欢消费的多样性，市场由销售不同种类商品的公司组成，并且（内部）规模收益递增。这意味着随着产量的增加，企业的平均生产成本下降。
该理论预测贸易成本可能对小型发展中经济体产生不成比例的负面影响。通常情况下，小型发展中经济体拥有规模收益不变的大规模的农业或自然资源部门，但只有小规模的制造业。相比之下，大型发达经济体拥有在规模收益递增的情况下运作的庞大的制造业。在这种情况下，贸易成本既导致贸易减少，又使制造业不成比例地迁往大的发达国家（“国内市场效应”）。与此同时，小的发展中国家集中在农业或自然资源部门。
解释这一结果的关键在于消费者对多样性的偏爱与规模收益递增之间的紧张关系。由于开放贸易和零贸易成本，发达国家的消费者同时购买外国和国内制成品，因为他们偏爱多样性。在所有条件相同的情况下，对多样性的偏爱导致更多的贸易。另一方面，由于可获得的市场规模和生产规模的扩大，发达国家的制造业企业具有成本优势。总而言之，发达国家的消费者更愿意购买成本较低的国内产品，而不是成本较高的国外产品。