什麼是經濟學中的"外部性"?大綱

The market for cigarettes has a negative externality on Consumption because consuming cigarettes imposes a cost on others not involved in the market for cigarettes in the form of second-hand smoke.

香菸市場對消費展現了負面的外部性，因爲它用二手菸的形式對其他人徵收了一種不會統計在香菸市場中的成本。

And that's what we'll talk about today: Externality.

而這就是我們今天要談的東西：外部性。

When making the claim that free, unregulated markets maximize the amount of value created for a society, e conomists either implicitly or explicitly assume that the actions and choices of producers and consumers in a market don't have any spillover effects onto third parties who are not directly involved in the market as a producer or a consumer.

當經濟學家聲稱自由、無監管的市場能最大化社會中的價值時，他們是在要麼有意、要麼無意地假設一件事，那就是生產者和消費者的一切行爲和選擇都不會對那些局外人造成任何外溢效應；這裏說的局外人指的是那些不直接以生產者或消費者身份參與該市場的人。

When this assumption is taken away, it no longer has to be the case that unregulated markets are value-maximizing, so it's important to understand these spillover effects and their impacts on economic value.

當這個假設被移除的時候，那不受監管的市場就不一定能夠帶來價值最大化了。所以，瞭解這些外溢效應以及他們對經濟價值的影響是十分重要的。

E conomists call effects on those not involved in a market externalities, and externalities vary along two dimensions.

經濟學家把這種對市場外的人造成的影響稱之爲外部性，而外部性會在兩個維度上變化。

First, externalities can be either negative or positive. Not surprisingly, negative externalities impose spillover costs on otherwise uninvolved parties, and positive externalities confer spillover benefits on otherwise uninvolved parties.

首先，外部性可以是負面的也可以是正面的。顧名思義，負面外部性會給未參與方帶來外溢成本，而正面外部性會給未參與方帶來外溢收益。

Second, externalities can be either on production or consumption. In the case of an externality on production, the spillover effects occur when a product is physically produced. In the case of an externality on consumption, the spillover effects occur when a product is consumed.

第二，外部性可能是生產端的，也可能是消費端的。生產端的外部性是在產品被生產時發生的。消費端的外部性是在產品被消費時發生的。

Combining these two dimensions gives four possibilities:

將這兩個維度聯合起來，我們就得到了4種可能性：

Negative Externalities on Production and Positive Externalities on Production

生產端的負面外部性和生產端的正面外部性

Negative externalities on production occur when producing an item imposes a cost on those not directly involved in producing or consuming the item.

如果一件產品在生產時會對市場外的人帶來成本，那這個產品就具有生產端的負面外部性。

For example, factory pollution is the quintessential negative externality on production, since the costs of pollution are felt by everyone and not just those who are producing and consuming the products that are causing the pollution.

比如，工廠的污染就是最典型的生產端的負面外部性，因爲污染會波及到所有人，而不僅僅是那些生產、消費污染源頭產品的人。

Negative Externalities on Consumption and Positive Externalities on Consumption

消費端的負面外部性和消費端的正面外部性

Negative externalities on consumption occur when consuming an item actually imposes a cost on others.

如果一件產品在消費時會對市場外的人帶來成本，那這個產品就具有消費端的負面外部性。

For example, the market for cigarettes.

比如，香菸市場。

Because the presence of externalities makes unregulated markets inefficient, externalities can be viewed as a type of market failure.

由於外部性會讓不受監管的市場變得低效，所以外部性可以被看做是一種市場失調。

This market failure, at a fundamental level, arises because of a violation of the notion of well-defined property rights, which is in fact a requirement for free markets to function efficiently.

這種失調，歸根結底是因爲產權的界定不夠嚴密；如果一個市場要高效運作，那這種界定就必須得嚴密。

This violation of property rights occurs because there is no clear ownership of air, water, open spaces, and so on, even though society is affected by what happens to such entities.

這些產權之所以不嚴密，是因爲空氣、水、空間等等的東西都沒有明確的所有人，但整個社會都會受這些東西影響。

When negative externalities are present, taxes can actually make markets more efficient for society.

當負面外部性存在時，稅收實際上能讓一個社會的市場變得更高效。

When positive externalities are present, subsidies can make markets more efficient for society.

當正面外部性存在時，補貼能讓市場變得更高效。