在生活中我们常常有这样的体会，如果一个地区盛产某种物品，且极有名，那么在本地区一般都只能买到该物品的次级品，而优质品大多用于了外销，在本地很难见到。经济学上把这种现象称作“优质苹果外运”。
关于这个问题的讨论，一般认为起源于1972年一位署名M.W.P的读者给《西雅图时报》写的一封信。这位读者抱怨华盛顿产的优质苹果都被运往欧洲或中东了, 当地市场上出售的苹果又小又难看。阿尔钦（Armen A Alchian）和艾伦（ William R. Allen ）对这个问题做了解释，该解释被称为“阿尔钦——艾伦定理”（the Alchian and Allen Theorem）。 但事实并非如此。
后来，芝加哥大学的两位教授John P. Gould和Joel Segall在《政治经济学杂志》（Journal of Political Economy）上发表了《运输成本的替代效应》（The Substitution Effects of Transportation Costs）一文、对这一解释提出质疑。问题正是出在这里，《运输成本的替代效应》的发表时间是1968年，比“阿尔钦——艾伦定理”的提出时间1972年早了四年，显然是矛盾的。
进一步查找有关文献，在阿尔钦和艾伦1964年所写的一本名为“University Economics”的教科书，以葡萄为例解释了优质品外运的现象。这是我们所查到的关于优质品外运的最早的解释。因此关于优质品应当始于此或之前，而不是前文所说的1972年是关于苹果的讨论。
看来，做学问也要坚持打破沙锅问到底。唯有如此，才能弄清事情的来龙去脉。
附“University Economics”教科书原文：
Suppose that grapes are grown in California and that it costs 5 cents a pound to ship grapes to New York whether the grapes are "choice" or "standard" (poorer) and that the total production of grapes is 50 percent "choice" and 50 percent "standard." Suppose further that in California the "choice" grapes sell for 10 cents a pound and the standard for 5 cents a pound; that is in California 2 pounds of "standard" and one pound of "choice" grapes sell for the same price. If grapes are shipped to New York, the shipping costs will raise the costs of "choice" grapes to 15 cents and of "standard" grapes to 10 cents. In New York the costs of "choice" grapes are lower relative to "standard" grapes (1.5 to 1) than in California (2 to 1). To buy 1 pound of "choice" grapes in New York would mean a sacrifice of 1.5 pounds of "standard" whereas in California it would cost two pounds of "standard." According to our law of demand, New Yorkers faced with a lower price of "choice" grapes relative to "standard" will consume relatively more "choice" grapes than Californians will. In California where "standard" grapes are cheaper relative to "choice" grapes a larger fraction of "standard" grapes will be consumed-and this is what actually happens.（Alchian, Armen A., and William R. Allen，1964：University Economics， Belmont，California：Wadsworth. p.75）