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In economics, a demerit good is a good or service whose consumption is considered unhealthy, degrading, or otherwise socially undesirable due to the perceived negative effects on the consumers themselves. It is over-consumed if left to market forces. Examples of demerit goods include tobacco, alcoholic beverages, drugs, gambling, junk food and prostitution. Because of the nature of these goods, governments often levy taxes on these goods (specifically, sin taxes), in some cases regulating or banning consumption or advertisement of these goods.

There is an important conceptual distinction between a demerit good and a negative externality. A negative externality occurs when the consumption of a good has measurable negative consequences on others who do not consume the good themselves. Pollution (due, for example, to automobile use) is the canonical example of a negative externality. Another example is cigarettes. It not only affects affects a deliberate or addicted smoker through damage to health, but also indirect smokers exposed to the smoke (second hand smoking). By contrast, a demerit good is viewed as undesirable because its consumption has negative effects upon the consumer.

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The market for merit goods is an example of an incomplete market. Merit goods have two basic characteristics:

Firstly, unlike a private good, the net private benefit to the consumer is not fully recognised at the time of consumption. Net private benefit is the utility from gained from consumption less any private cost incurred, and equates to net consumers surplus. In the case education, which is widely considered to be a merit good, pupils and students cannot possibly know the specific private benefit to them of getting good grades at school, college or university. They will be well aware of the sacrifice required to study, but will not know the benefits to them in terms of a future job, salary, status and skills. Therefore, with education, as with other merit goods, there is a significant information failure in terms of expected benefits.

Secondly, while consumption of a merit good also generates an external benefit to others, from which society gains, this is unlikely to be known or recognised at the point of consumption. Given that decisions to consume are driven by self-interest, it is unlikely that this external benefit will be taken into account when the consumer of a merit good evaluates its worth. For example, an individual student is generally not motivated to study hard in order to benefit others later in life, although everyone associated with them will benefit from their education in some way. Beneficiaries include future employers and all those who consume the products supplied their employer, their family, and friends. The better job they obtain, the more tax they will pay, and the greater the benefit to those who receive welfare benefits and transfers. However, putting a value on these external benefits is impossible, especially at the point of learning.

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A positive externality (also called "external benefit" or "external economy" or "beneficial externality") is the positive effect an activity imposes on an unrelated third party. Similar to a negative externality, it can arise either on the production side, or on the consumption side. - Example a new airport is a posative externality because the pollution is out weighed by the marginal private cost is greater then marginal social cost.

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Monopoly power

Markets may fail to control the abuses of monopoly power.

Missing markets

Markets may fail to form, resulting in a failure to meet a need or want, such as the need for public goods, such as defence, street lighting, and highways.

Incomplete markets

Markets may fail to produce enough merit goods, such as education and healthcare.

Demerit goods

Markets may also fail to control the manufacture and sale of goods like cigarettes and alcohol, which have less merit than consumers perceive.

Negative externalities

Consumers and producers may fail to take into account the effects of their actions on third-parties, such as car drivers, who may fail to take into account the traffic congestion they create for others. Third-parties are individuals, organisations, or communities indirectly benefiting or suffering as a result of the actions of consumers and producers attempting to pursue their own self interest.

Property rights

Markets work most effectively when consumers and producers are granted the right to own property, but in many cases property rights cannot easily be allocated to certain resources. Failure to assign property rights may limit the ability of markets to form.

Information failure

Markets may not provide enough information because, during a market transaction, it may not be in the interests of one party to provide full information to the other party.

Unstable markets

Sometimes markets become highly unstable, and a stable equilibrium may not be established, such as with certain agricultural markets, foreign exchange, and credit markets. Such volatility may require intervention.

Inequality

Markets may also fail to limit the size of the gap between income earners, the so-called income gap. Market transactions reward consumers and producers with incomes and profits, but these rewards may be concentrated in the hands of a few.

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A negative externality (also called "external cost" or "external diseconomy") is an economic activity that imposes a negative effect on an unrelated third party. Example - (We burn coal to give us energy but this produces air pollution that effects third parties such as children who don't choose to use the energy but still are affected.)

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Market failure is any situation where the allocation of resources by a free market is not efficient. These situations from society's viewpoint, could be improved on if resources were allocated differently. Market failure is most often associated with market power, asymmetric information and externalities.