来自《耶鲁大学开放课程：金融理论 第4集 第4集》

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  Because B's utility depends only on what B's consuming,
  因为B的效用取决于B的消费
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  not what A's doing.
  而非A的所作所为
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  So there's no place in this model.
  但是模型中没有空间
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  It's too narrow to include externalities,
  模型涵盖范围有限而无法包含外部效应
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  so it doesn't capture the fact that he,
  因此 模型不包含这样一个事实
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  by smoking, might make her sick by smoking so many cigarettes
  A过量吸烟会使B染病
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  and that's not in here.
  这不在模型中
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  And if you put the idea that B's utility, her utility might
  如果你把这个想法 即B的效用可能
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  depend on what he's doing and not what she's doing
  取决于A所做的而不是B所做的
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  the theorem won't be true.
  理论将有失真实
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  So there's a reason to tax pollution, and to tax cigarettes
  因此就会有理由对污染 烟草征税
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  and all those things because of externalities.
  都缘于外部效应