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In the thought experiment, a hypothetical being is proposed who receives as much or more utility from each additional unit of a resource he consumes as the first unit he consumes. In other words, the utility monster is not subject to diminishing marginal returns with regard to utility, but instead experiences constant marginal returns, or even increasing marginal returns.
Since ordinary people receive less utility with each additional unit consumed, if the utility monster existed, the doctrine of utilitarianism would justify the mistreatment and perhaps annihilation of everyone else, according to Nozick's argument.
In his words:
- Utilitarian theory is embarrassed by the possibility of utility monsters who get enormously greater sums of utility from any sacrifice of others than these others lose . . . the theory seems to require that we all be sacrificed in the monster's maw, in order to increase total utility.
This thought experiment attempts to show that utilitarianism is not actually egalitarian, even though it appears to be at first glance.
Some ways of aggregating utility, such as based on Rawls' maximin or difference principle, can circumvent the conclusion that all units should be given to the utility monster. In particular, maximin sets a group's utility as that of the being with least utility. Thus, giving units to the utility monster fails to change the group's utility unless the utility monster has the least utility. Even if the utility monster has the least utility, maximin would only prefer allocating units to the monster until it catches up with the member that has next-to-least utility.
Social implications 
Capitalist economics 
The utility monster has been identified directly with capitalism, and particularly the neoliberal doctrine that free market relations are the best way to constitute society. The discipline of economics is related because it identifies "utility" as a quantity that can be optimized across human beings. According to critics, these ways of thinking produce decisions that are selfish, shortsighted, and destructive to the many while benefiting the few.
The utility monster has been invoked in debates about population. Derek Parfit's mere addition paradox suggests that additional humans would add to total happiness, even if expanding population decreases average happiness. Opposite reasoning yields the "repugnant conclusion" that the world would be better off with one extremely happy person. Parfit suggests that Nozick's utility monster is unrealistic because one being could not experience more utility than millions of other beings put together.
See also 
- Robert Nozick, Anarchy, State, and Utopia (1974)
- Mark A. Lutz and Kenneth Lux, "New directions in humanistic economics or How to overcome the utility monster", Forum for Social Economics 14(1), 1984.
- Derek Parfit, "Overpopulation and the quality of life", in The Repugnant Conclusion, J. Ryberg and T. Tännsjö, eds., 2004.