When negative externalities in consumption occur, increasing the consumption of a good will always result in a higher marginal social benefit than the marginal private benefit for consumers.

Negative externalities in consumption happen when the use of a good by one person negatively affects others who are not involved in that transaction. For example, if someone smokes in a public place, the smoke can harm the health of people nearby, even though they did not choose to smoke. When more people consume the good, like cigarettes, the harm to others increases, which means the overall social benefit decreases, even if the individual consumer feels they are benefiting. As a result, the extra benefit that society loses from this harm is greater than the benefit the individual consumer receives, leading to a situation where the marginal social benefit is lower than the marginal private benefit. Therefore, increasing consumption in such cases does not lead to a higher social benefit; it actually creates more harm for others.