In economics, an externality is a cost or benefit that is imposed on a third party who did not agree to incur that cost or benefit. For the purpose of these statements, overall cost and benefit to society is defined as the sum of the imputed monetary value of benefits and costs to all parties involved. The concept of externality was first developed by economist Arthur Pigou in the 1920s. Air pollution from motor vehicles is an example of a negative externality. The costs of the air pollution for the rest of society is not compensated for by either the producers or users of motorized transport.