李嘉图经济学
李嘉图出生于1772年，后来靠做证券经纪人和贷款经纪人致富。 在27岁时，他读了亚当斯密的"国富论"，并被当中的经济学原理所激发。他的主要经济学思想都集中在1817年出版的政治经济学和税收原理当中。这本书提出了一系列的理论，他们后来成了马克思的资本论和马歇尔主义经济学共同的理论基础。这里所讲的理论，包括地产（土地）收益理论、价值的劳工理论以及最重要的比较优势理论.
李嘉图在读过亚当斯密后十年写了他的第一篇经济学文章，最终这篇"金条之争"凭借其关于19世纪英格兰通货膨胀的理论而为李嘉图在经济学界树立了名声。这个理论后来被称为货币主义,主要内容是过度的货币量导致通货膨胀。 李嘉图也为古典经济学的创建做出了巨大贡献， 因为他积极为不受政府法律或壁垒限制的自由贸易 和 自由竞争摇旗呐喊。
另一个让李嘉图出名的概念出自他的"关于玉米低价格对于股票收益影响的随笔"，这就是著名的边际效应递减法则 (李嘉图, 经济学随笔, Henderson 826).所谓的边际效应递减法则说的是如果对一个生产要素提供更多的数量而对另外一个生产要素保持数量不变，则总产量created by the extra units will eventually get smaller to a point where overall output will begin to fall ("Diminishing Returns").
举一个例子，考慮一個只有兩資源--勞力和土地的簡單農場。假設該農場有一百畝土地及一名工人(勞力資源)。這個地-工組合可達到一定程度的產出。 如果我們增加土地且勞動力不變，該工人對每畝地的照料程度就會下降(假設其他條件皆不變)。所以，產出可能會增加，但單位土地的產量可能下降。如果我們繼續增加該名工人須照料的土地，最終會因土地過剩而導致工人分身乏術，總產量開始下滑(及分配給各畝田的勞動時數減少) 。這就是典型的邊際效應。 This is the typical stylized result of increasing one productive input while holding the others constant (versus increasing all inputs, generating economies of scale).
李嘉图反对关税以及其他对于国际贸易的限制. 李嘉图发展了著名的比较优势理论 (Henderson 827, Fesfeld 325). 根据华盛顿国际贸易理事会的定义，比较优势是指相对于其他国家，某国能够以相对于其他商品较低的价格生产某种商品。在"经济学原理"一书中, 李嘉图表示，比较优势是一种specialization technique used to create more efficient production (52) and describes opportunity cost between producers (53). With perfect competition  and undistorted markets, countries tend to export goods in which they have a comparative advantage ("Comparative Advantage").
在举一个例子，设想两个国家同时生产卡片和pencils，并且使用同样的时间生产一份产品（见表格）. 国家甲can make 4 pencils if they specialize just in pencils at the expense of one card, but this country can also make ¼ of a card at the expense of one pencil. 同样的逻辑适用于国家乙: 如果国家乙makes only pencils, it will make 2 pencils at the expense of 1 card.。如果国家乙specializes only in cards, it will make ½ of a card at the expense of a pencil. 比如，,国家甲在pencils上面相对于国家乙有比较优势(4 pencils to 2 pencils), 然而国家乙在cards上相对于国家甲有比较优势(½ of a card to ¼ of a card). 在李嘉图的比较优势的概念中，这两个国家应该专注于他们能够做的更好的行业。根据"经济学财富百科全书"的说法，李嘉图的比较优势理论是"今天大多数经济学家信奉自由贸易的主要理论基础" (827).
尽管大卫 李嘉图是19世纪人物， 很多人在日常经济学中还在引用他的工作成果. 李嘉图的rent理论主要包括农民和地主的农业模式。由于高生产力的土地能够生产更多的农作物，市场将支付相同价额购买有利和不利的土地上种植的作物，农民愿意付出更多钱来买高生产力的土地来种植更多的作物挣钱（Henderson 827).
李嘉图也有另一个有影响力的理论：最低工资。他知道如果较大幅度的人口增长，就业需求就会增加，使得工资下降到一个水平上，因为很多人愿意采取低工资的工作，从而得以生存（圣克莱尔9，Fusfeld 325）。这个观察最低工资的工作成果，尤其是今天显得极为正确，只要看一看与执行最低工资法相关的争议便明白了。。在他的著作"政治经济学和税收原理"中,李嘉图说，工作提供更多的工资给我们珍惜的工作，而不是那些不珍惜的。对李嘉图而言,价值和生产成本有很大关系，后者包括工资和利润 (St. Clair 27) 并且，你支付多少钱给工人影响产品的价格。他还认为the value of a product was related to the quality of labor necessary for the production (Principles of Political 5). An example of this would be paying a slightly higher price for an item that is handmade, rather than being manufactured. Though this is true, 李嘉图 also thought the labor or machine itself should be considered when selling an item and that a little of every item should be priced to include this factor of labor (St. Claire 24). 李嘉图解决我们今天的经济世界所面临的许多问题，如最低工资和租金(Fusfeld 325).也许这些问题对我们非常重要，正如他们在19世纪的时候一样，这是为什么大卫李嘉图的经济学理论是现代经济学的重要组成部分。
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