外部性理論의 圖解

Diagrammatic Exposition of a Theory of Externality: à la Samuelson

Suppose an economy consisting of J consumers consuming the two commodities, X and Y. A ‘generalized’ definition of externality is as follow: Exterality exists where A’s X consumption causes a non-compensable spill-over effect over B as defined as {XB = XB (XA)}. This defintion turns into a conventional one by substituting the parenthesized expression by {XB = XA}. According to the definition by Samuelson (l955), the characteristic of non-depletion exists when {X=XA=XB} holds. The existence of externality as conventionally defined, therefore, indicates as a corollary the existence of non-depletion. Ever since Samuelson(l954) defined public goods as commodities characterized by non-depletion, non-depletion has been commonly perceived as a unique nature of public goods. By demonstrating that the characteristic of non-depletion is shared by external goods as well as public goods, this paper attests that such a common sense is a misconception. The diagrammatic exposition of public goods’ efficiency condition by Samuel-son(l955) has been taken for applying exclusively for public goods. This paper verifies that, on the contrary, the same diagrams exposes external goods’ efficiency conditions as well. In this vein the present paper is expected to manifest the true nature of externality theory and clarify the relationship between public goods and external goods.