5.5 Implications of the MRS Condition
In the last section we examined the important idea that observation of demand behavior tells us important things about the underlying preferences of the consumers that generated that behavior. Given sufficient observations on consumer choices it will often be possible to estimate the utility function that generated those choices.
But even observing one consumer choice at one set of prices will allow us to make some kinds of useful inferences about how consumer utility will change when consumption changes. Let us see how this works.
In well-organized markets, it is typical that everyone faces roughly the same prices for goods. Take, for example, two goods like butter and milk. If everyone faces the same prices for butter and milk, and everyone is optimizing, and everyone is at an interior solution... then everyone must have the same marginal rate of substitution for butter and milk.
This follows directly from the analysis given above. The market is offering everyone the same rate of exchange for butter and milk, and everyone is adjusting their comsumption of the goods until their own "internal" marginal valuation of the two goods equals the market's "external" valuation of the two goods.
Now the interesting thing about this statement is that it is independent of income and tastes. People may value their total consumption of the two goods very differently. Some people may be consuming a lot of butter and a little milk, and some may be doing the reverse. Some wealthy people may be consuming a lot of milk and a lot of butter while other people may be consuming just a little of each good. But everyone who is consuming the two goods must have the same maginal rate of substitution. Everyone who is consuming the goods must agree on how much one is worth in terms of the other: how much of one they would be willing to sacrifice to get some more of the other.
The fact that price ratios measure marginal rates of substitution is very important, for it means that we have a way to value possible changes in consumption bundles. (중략)
The point is that, since prices measure the rate at which people are just willing to substitue one good for another, they can be used to value policy proposals that involve making changes in consumption. The fact that prices are not arbitrary numbers but reflect how people value things on the margins is one of the most fundamental and important ideas in economics.
Hal R. Varian, 「Intermediate Microeconomics」p.g 85-86
한계대체율이 가격과 같으므로 가격이 생산활동과 정책입안의 기준이 된다는 설명은 제법 설득력이 있다. 그런데 나는 아직도 완전경쟁시장(주어진 가격, 동질적 재화 등)의 가정으로부터 도출된 가격이 존재하는가 하는 고전적 문제제기를 되새기게 된다. 완전경쟁시장이 존재하지 않는다면 그 시장으로부터 도출된 가격을 기준으로 정책을 입안하는 것은 현실과 동떨어진 행동이 아닌가?
베리안은 미시경제이론들을 설명하는 중간중간에 꾸준히 해당 이론의 정책적 시사점을 설명하고 있다. 매력적인 설명이고, 이론적으로 체계적이며 분명하다. 그럼에도불구하고, 경쟁시장의 존재를 믿을 수 없는 나는, 가격을 기준으로 정책을 수행하는 사람들을 가격의 신도, 시장의 교조주의자들로 볼 수 밖에 없다. 그중에 똑똑한 사람들이 많다는 건 슬픈 일이다.