Public goods: rivalry and excludability

Public goods are goods or services that are non-excludable and non-rivalrous in nature. Non-excludability refers to the fact that it is difficult or impossible to exclude someone from using the good or service once it has been provided. Non-rivalry means that consumption of the good or service by one person does not reduce the amount available for others.

However, there are two different types of public goods. The first type is non-excludable but rivalrous, such as a fishery. If too many people fish in the same area, the resource becomes depleted, making it less available to others. The second type is both non-excludable and non-rivalrous, such as air or national defense. The use of these goods or services by one person does not impact their availability to others, and it is difficult or impossible to exclude someone from using them.

In summary, the two key characteristics of public goods are non-excludability and non-rivalry, but there are different types of public goods that may have some degree of rivalry or exclusivity.