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The thought experiment
A hypothetical being is proposed who receives much more utility from each unit of a resource they consume than anyone else does. For instance, eating a cookie might bring only one unit of pleasure to an ordinary person but could bring 100 units of pleasure to a utility monster. If the utility monster can get so much pleasure from each unit of resources, it follows from utilitarianism that the distribution of resources should acknowledge this. If the utility monster existed, it would justify the mistreatment and perhaps annihilation of everyone else, according to the doctrine of utilitarianism. For the utility monster, the pleasure they receive outweighs the suffering they may cause.
There is no way of aggregating utility which can circumvent the conclusion that all units should be given to a utility monster, because every different system has a monster and defeating one utility monster creates another. For example, in Rawls' maximin or difference principle, maximin sets a group's aggregate 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.[clarification needed] 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. This would defeat the "happy" utility monster of average utility. But if the person who has the least utility gains only a tiny amount of utility from each unit of resources, they may never catch up with the next person, so they can similarly consume all of the resources in the world. It can be shown that all consequentialist systems based on maximizing a global function are subject to utility monsters.
Robert Nozick, philosopher in twentieth century America, coined the term Utility Monster in response to Jeremy Bentham's philosophy of Utilitarianism. Nozick proposed that accepting the theory of Utilitarianism causes the necessary acceptance of the condition that some people would use this to justify exploitation of others. An individual (or specific group) would claim their entitlement to more "happy units" than they claim others deserve, and the others would consequently be left to receive less "happy units."
Nozick deems these exploiters 'Utility Monsters' (and for ease of understanding, they might also be thought of as Happiness Hogs). Nozick poses Utility Monsters justify their greediness with the notion that compared to others, they experience greater inequality or sadness in the world, and deserve more happy units to bridge this gap. People not part of the Utility Monster group (or not the Utility Monster individual themselves) are left with less happy units to be split among the members. Utility Monsters state that the others are happier in the world to begin with, so they would not need those extra happy units to which they lay claim anyway.
Commentary by Nozick
In the words of Robert Nozick:
- 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.
The utility monster has been invoked in debates about free speech. Advocates for laws against hate speech, flag-burning, and blasphemy have been accused of becoming utility monsters in order to increase society's willingness to support their policies.
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 misleading because it appeals to our intuitions about a being which experiences more than a million times the utility of a very well-off ordinary person, which is, he thinks, inconceivable. The implication is a more common-sense continuous scale of happiness change, from great to nil, based on scarcity of units, the happiness increasing from an additional unit of resource only inversely proportional to the existing pool of units.
The reason this can come to be, and the reason the Utility Monster is a condition of Utilitarianism in effect, is because the philosophy necessarily begs the question of how to measure happiness. A person can be in much grief, but there is no physical way to measure the lack of happiness they experience, and whether this is greater or less than a person who is enduring a different pain, like physical torture. Rephrased, this brings to light the question of which person is more deserving and which person is less deserving of happiness units based on life experiences. Individuals must take other's word regarding how much happiness they each possess, and the happiness they should therefore be able to lay claim. It is a common idea among people that hurt individuals deserve compensation for their pain. Yet Nozick's Utility Monster would take advantage of this reward process, by proclaiming their pain is the greatest and most deserving of reward.
Kuznicki argues the reverse is true. According to Kuznicki, this proposed justification negatively affects society, because people's demand for equal payment for life's pain creates these Utility Monsters. One such group he suggests comes in the form of people who seek political correctness. He states that these folks butcher other people's right to free speech, under the pretense that it causes their group (or their individual) pain. He states that it is unjustifiable that pain one causes another is greater than someone else's. He thus provides the example of censorship, where if one person finds a certain censorship offensive, while others do not, who's to say that the offended person's hurt is worthy of a law of censorship of that material to be created? Specifically,
"If 'feelings of upset' are to be taken into account in shaping our laws, why do my feelings, and the feelings of other libertarians, always count for nothing?" 
- Kennard, Frederick (March 20, 2015). Thought Experiments: Popular Thought Experiments in Philosophy, Physics, Ethics, Computer Science & Mathematics (First ed.). AMF. p. 322. ISBN 9781329003422.
- Nozick, Robert (1974). Anarchy, State, and Utopia. p. 41.
- Kuznicki, Jason (November 16, 2009). "Attack of the Utility Monsters: The New Threats to Free Speech" (PDF). Cato Institute. Cato Institute. Retrieved October 15, 2016.
- Derek Parfit, "Overpopulation and the quality of life", in The Repugnant Conclusion, J. Ryberg and T. Tännsjö, eds., 2004.