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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.
香烟市场对消费展现了负面的外部性，因为它用二手烟的形式对其他人征收了一种不会统计在香烟市场中的成本。
And that's what we'll talk about today: Externality.
而这就是我们今天要谈的东西：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.
当经济学家声称自由、无监管的市场能最大化社会中的价值时，他们是在要么有意、要么无意地假设一件事，那就是生产者和消费者的一切行为和选择都不会对那些局外人造成任何外溢效应；这里说的局外人指的是那些不直接以生产者或消费者身份参与该市场的人。
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.
当这个假设被移除的时候，那不受监管的市场就不一定能够带来价值最大化了。所以，了解这些外溢效应以及他们对经济价值的影响是十分重要的。
E conomists call effects on those not involved in a market externalities, and externalities vary along two dimensions.
经济学家把这种对市场外的人造成的影响称之为外部性，而外部性会在两个维度上变化。
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，外部性可以是负面的也可以是正面的。顾名思义，负面外部性会给未参与方带来外溢成本，而正面外部性会给未参与方带来外溢收益。
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，外部性可能是生产端的，也可能是消费端的。生产端的外部性是在产品被生产时发生的。消费端的外部性是在产品被消费时发生的。
Combining these two dimensions gives four possibilities:
将这两个维度联合起来，我们就得到了4种可能性：
Negative Externalities on Production and Positive Externalities on Production
生产端的负面外部性和生产端的正面外部性
Negative externalities on production occur when producing an item imposes a cost on those not directly involved in producing or consuming the item.
如果一件产品在生产时会对市场外的人带来成本，那这个产品就具有生产端的负面外部性。
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，工厂的污染就是最典型的生产端的负面外部性，因为污染会波及到所有人，而不仅仅是那些生产、消费污染源头产品的人。
Negative Externalities on Consumption and Positive Externalities on Consumption
消费端的负面外部性和消费端的正面外部性
Negative externalities on consumption occur when consuming an item actually imposes a cost on others.
如果一件产品在消费时会对市场外的人带来成本，那这个产品就具有消费端的负面外部性。
For example, the market for cigarettes.
比如，香烟市场。
Because the presence of externalities makes unregulated markets inefficient, externalities can be viewed as a type of market failure.
由于外部性会让不受监管的市场变得低效，所以外部性可以被看做是一种市场失调。
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.
这种失调，归根结底是因为产权的界定不够严密；如果一个市场要高效运作，那这种界定就必须得严密。
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.
这些产权之所以不严密，是因为空气、水、空间等等的东西都没有明确的所有人，但整个社会都会受这些东西影响。
When negative externalities are present, taxes can actually make markets more efficient for society.
当负面外部性存在时，税收实际上能让一个社会的市场变得更高效。
When positive externalities are present, subsidies can make markets more efficient for society.
当正面外部性存在时，补贴能让市场变得更高效。
