Buyers and sellers neglect the external effects of their actions when deciding how much to demand or supply.

External Effects

In economics, externalities or external effects occur when the actions of individuals or firms have positive or negative impacts on third parties not directly involved in the transaction.

Demand

Refers to the quantity of a good or service that consumers are willing and able to purchase at different prices at a given time.

Supply

The total amount of a specific good or service that is available to consumers at a given price level and over a specific period.