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The Approach: Utilitarianism-Based Cost-Benefit Analysis
Benatar argues from a Utilitarianism-based cost/benefit position, especially from a hedonistic (pleasure-based) "calculus", imperfect and subjective that may be. In short, he claims that if the pleasures of life are not worth the inevitable pains that life subjects us to, then humans don’t deserve to be born. He asserts this position based on a supposed “asymmetry” that exists in the very real-world nature of life, and perhaps even in the theoretical nature of life itself: A nonexistent person will not suffer, even as he or she can’t enjoy life’s benefits; yet an existing person cannot help but suffer due to the very nature of human living itself and how the world operates, even as he or she can enjoy at least some of life's benefits. Furthermore, he states that even if there is a moral duty to bring humans into existence, other moral considerations may override the procreative duty. Most particularly among these rights is the right of the unborn not to suffer as a result of being brought into existence.
Criticism of the Benatarian Approach
However, antinatalist blogger Chip Smith finds this approach incomplete at best. Even worse, he finds in it a serious ethical blind spot, namely that it seems to permit or possibly even mandate the murder of individuals and genocide against all humanity - on the grounds that murder/genocide would forever eliminate any possibility of suffering for those not yet conceived. Benatar himself saw such a possible derivation and presents as a counter to it arguments about death from Stoic philosophers. However, Smith found his counter unconvincing, thereby still leaving the cost-benefit calculus vulnerable to such derivations as far as he is concerned.
Therefore, Smith explores a different Libertarian approach, one based on Kantian deontological (duty/obligation-based) ethics, “proprietary rights” (i.e. “the right of self-ownership”) and its corollary “Axiom of Non-Aggression"; the latter two notions as expounded by Libertarian philosopher Murray Rothbard. From these three bases, Smith extracts the notion that conceiving a child is an act of aggression against that child. It’s aggression because of two facts: (a) the parent’s full awareness that life inevitably has hardships and suffering, especially the non-trivial probability of the child enduring great suffering at some point (and certainly will die one day) and (b) the impossibility of consent by the child to be born into a world such as this. Thus, he shows there is another Libertarian route to antinatalism, one not vulnerable to extrapolations to justifications of killing people.
Closely related to the death problem, Smith also addresses the issue of childbirth from an Abrahamic religious perspective in fair detail (though he himself is an atheist). According to Christianity, Islam, and even some forms of Orthodox Judaism, any one random person has a very non-trivial probability of suffering a terrible afterlife. From this, it follows that there's even less justification to have children and by the same token a greater obligation for religious people to adopt antinatalism than non-religious people have. This resonates quite strongly when one recalls the cliche "a fate worse than death", which is all too literally true in Abrahamic theologies and in some other ones as well.
In conclusion, while Smith did expose an ethical blind spot in Benatar's reasoning, it's highly unlikely to to fatally wound the cost-benefit approach, except in its purest (i.e. most one-dimensional) form; nor does Smith even seem to intend to completely overthrow his approach. When all is said and done, Benatar's main contribution is to bring into greater conscious awareness the need to weigh the costs and benefits of bringing any one person into existence. This is certainly a vital component of making a properly-informed decision about whether one should have children or not. As such, Better to Have Never Been certainly serves a worthwhile purpose.
 Smith himself has problems with all three notions; to the point of thinking that the notion of "rights" in and of itself is a mere abstraction, let alone deontology-based ones. It is for this reason I changed the sentence from "Smith advocates..." to "Smith explores ..."
 This is Smith's chief objection to antinatalism - independent of the argument he presents. From the fact that everyone dies, he derives the idea that childbirth is in a moral sense an act of negligent homicide at best and outright murder at worst.