In this post I try to give some reasons why there are so many utilitarians among smart people, from personal experience reading things on the net. The point of this post is not to criticise or defend utilitarianism, just offer a presentation of the reasons why some people, I think, tend to favour it. I begin with a quote from Loren Lomasky, defining what we are talking about. You can also go to the SEP.
For present purposes, a theory is said to be (more or less) utilitarian to the extent it satisfies the following eight conditions, ordered roughly in descending importance
1. Consequentialism. The rightness or wrongness of actions, desirability or undesirability of rules, admirability or unattractiveness of dispositions of character, and similar appraisals are ultimately a function of the nature of the consequences they actually, probably, or for the most part generate. It may be more perspicuous to phrase the condition negatively: nothing other than the quality of consequences is an ultimate determinant of moral value.
2. Comparativeness. Evaluations are comparative. The recommended action, rule, disposition, etc. is that which achieves a consequential payoff higher than that of feasible alternatives. So, for example, an action that generates considerable misery will be the right action should it be the case that all other actions open to the agent would produce yet greater misery; one that yields joy for thousands is wrong if another course yields joy for millions.
3. Individualism. The units of utility are reckoned in the coin of the well-being of distinct individuals. In classical versions of utilitarianism these units are pleasure/absence of pain; subsequent formulations fasten on preference-satisfaction as basic.
4. Universalism. Everyone’s well-being matters. This is laudably simple. Not so simple, however, is nailing down the scope of ‘everyone.’ Does it include/exclude non-human animals, potential but non-actual persons, and so on? Different thinkers advance different scope judgments.
5. Aggregationism. Overall social assessments (also known as “social welfare functions”) are an additive function of the well-being of all affected parties. Each is to count equally with every other. However, because the capacity of some to experience pleasure/satisfy important preferences may be greater than that of others, it does not follow that these “equal” parties will enjoy similar influence within the calculus.
6. Determinacy. There exists, at least in principle, a decision-procedure for determining between any actions, rules, etc. which is to be preferred. That is, there cannot exist a situation in which there does not exist among an agent’s/legislator’s alternatives a right one.
7. Interpersonal Comparability. There exists some rationally ascertainable metric, not necessarily fine-grained, in terms of which benefits/harms to the affected parties can be weighed against each other pursuant to aggregation.
8. Cardinality. Well-being measures of individuals and across individuals can be arrayed not merely on a greater/less than scale (ordinal comparison) but on an absolute scale (compare to height and weight).
The classical utilitarians satisfy all eight of the conditions, although it needs to be said Mill’s addition to the hedonic mix of a qualitative dimension of higher/lower pleasures may render cardinality problematic. A G. E. Moore-style utilitarianism that recognizes valuable 3 items other than subjective conscious states deviates modestly from the utilitarian paradigm; yet further toward the periphery but still recognizably utilitarian in impulse are Kaldor-Hicks welfare economics and cost-benefit analysis. Paretianism is situated a further hop from the center.
Reversing chronology, Hume and Smith are, if not utilitarians, then proto-utilitarians, especially insofar as their theories appraise economic institutions. It is, then, not unduly strained to give them early position on a line that extends through Bentham, the Mills, and beyond.
The Decline of Laissez Faire Utilitarianism, Loren E. Lomasky (unpublished, unavailable)
One peculiar feature about actually existing utilitarians is that many of them don't believe there are moral facts. Incidentally, that's what Michael Huemer  criticised Peter Singer once for: His metaethics conflict with his ethics . The general attitude seems to be one of a vague "Well, all ethics are arbitrary, but at least here you can use numbers, or you can apply it consistently". Here there are some reasons why utilitarianism seems so compelling to many people, from the point of view of utilitarians themselves. Listing doesn't imply agreement or disagreement.
- It seems there are no plausible alternatives
- It's a natural extension of one person instrumental reasoning (aka expected subjective utility maximisation)
- There are lots of smart people endorsing it!
- It's simple: One easy to remember principle applied to everything in a consistent way
- It enables you to easily plug in results from economics, and apply it to economic policy
- Utilitarians (e.g. Bentham) were way ahead of their time in moral beliefs 
- Economic freedom
- Freedom of expression
- Separation of Church and State
- Women's Rights
- Right to divorce
- Abolition of slavery
- Animal Rights
- Abolition of death penalty
- Abolition of corporal punishment
- Prison reform
- Decriminalization of homosexual acts
- Neuroscience has proven that non-utilitarian ethics are emotion based, while utilitarianism is reason based 
- It seems to account for many intuitions of right/wrong such as the ones listed for the case of Bentham (plus justifies economic redistribution)
- It takes into account consequences, unlike hard deontology
- It takes into account risk, and makes proababilistic reasoning possible, and clear
- Everyone cares about happiness/welfare/preference satisfaction
- It is universal, it can be applied to any ethically relevant being, in any situation
- It is symmetric, the one who is doing the reasoning is not 'special'. Everyone counts equally and stands in the same relation to others.
- It is functional: By definition, unless there is a logical incoherence in the formulation of utilitarianism, if it doesn't work (according to utility), the utilitarian will not endorse it.
 "Peter Singer's stated view about the nature of ethics is both non-cognitivist and Humean. This meta-etical view, however, clashes with the rest of is philosophy on three levels. First, it clashes with his view of what we are ethically required to do; second, it leaves him with no plausible account of why we should be moral given his conception of what morality demands; and third, it clashes with his methodology [based in intuitions, (this comment is mine)] in ethics." Huemer, M. Singer's Unstable Meta-Ethics in Schaler, J. A. (2009). Peter Singer under fire: the moral iconoclast faces his critics. Open Court Publishing.
 Singer is[2.1] not a nihilist/error theorist, but a non-cognitivist. For him, a moral judgement is a sort of prescription, that has to be coherent, consistent, and universalizable.
[2.1] Or was. He changed his ethics from preference utilitarianism to utilitarian hedonism in his latest book with Katarzyna de Lazari-Radek, The Point of View of the Universe. Maybe he also changed his meta-ethics. This is little known: many people think Singer is still a preference utilitarian.
 According to Julia Markovits
 See Greene, J. (2014). Moral tribes: emotion, reason and the gap between us and them. Atlantic Books Ltd. or listen to this podcast
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