An externality occurs whenever the actions of
one party make another party worse or better off, yet the first party neither
bears the costs nor receives the benefits of doing so.
หากการกระทำของฝ่ายหนึ่งทำให้เกิดผลกระทบกับอีกฝ่ายหนึ่ง ดดยผลกระทบนั้นทำให้อีกฝ่ายแย่ลง
A negative externality occurs when an individual or firm making a decision does not have to pay the full cost of the decision. If a good has a negative externality, then the cost to society is greater than the cost consumer is paying for it. Since consumers make a decision based on where their marginal cost equals their marginal benefit, and since they don't take into account the cost of the negative externality, negative externalities result in market inefficiencies unless proper action is taken.