All arguments used to justify abortion also logically justify infanticide. Pro-abortion arguments such as that rights are acquired rather than inherent, as well as the numerous criteria that have been suggested to qualify for rights (such as achieving a level of rationality or certain physiological characteristics), all apply just as equally to newborn infanticide as they do to abortion.
This means that a defence of abortion logically entails a defence of infanticide. You cannot choose to defend abortion and not defend infanticide without contradiction. This is a matter of fact that influential pro-abortion philosophers explicitly accept. Here are some examples.
In Practical Ethics (1979), Peter Singer readily admits to the logical connection between arguments for abortion and infanticide:
liberals usually hold that it is permissible to kill an embryo or fetus but not a baby. I have argued that the life of a fetus (and even more plainly, of an embryo) is of no greater value than the life of a nonhuman animal at a similar level of rationality, self-awareness, capacity to feel and so on, and that because no fetus is a person, no fetus has the same claim to life as a person. Now we have to face the fact that these arguments apply to the newborn baby as much as to the fetus. A week-old baby is not a rational and self-aware being, and there are many nonhuman animals whose rationality, self-awareness, capacity to feel and so on, exceed that of a human baby a week or a month old. If, for the reasons I have given, the fetus does not have the same claim to life as a person, it appears that the newborn baby does not either. Thus, although my position on the status of fetal life may be acceptable to many, the implications of this position for the status of newborn life are at odds with the virtually unchallenged assumption that the life of a newborn baby is as sacrosanct as that of an adult… I do not regard the conflict between the position I have taken and widely accepted views about the sanctity of infant life as a ground for abandoning my position.
In Abortion and Infanticide (1983), Philosopher Michael Tooley acknowledges that pro-choice arguments are pro-infanticide arguments and he knows that this is a "difficult issue" to accept:
Most current discussions of abortion tend to treat it in isolation from the question of the morality of infanticide. One of the central contentions to be advanced here is that it is very difficult indeed to arrive at a defensible position on abortion unless one is prepared to come to terms with the difficult issue of the moral status of infanticide.
David Boonin's 1998 book A Defense of Abortion evaluates various arguments in terms of how "attractive" they are to those wanting to defeat rights-based arguments against abortion. Having rejected one suggestion, he considers whether to defend abortion by defending infanticide, but decides that this is not "attractive" since to acknowledge that abortion is on a par with infanticide would delegitimise it:
A defender of abortion …. could agree that you and I have a right to life but deny that newborn infants do. If newborn infants do not have a right to life, then it will again be a simple matter to establish that fetuses lack such a right, and the rights-based argument against abortion will again be defeated. This suggestion is likely to strike most readers as hardly more attractive than the first. In the popular debate about abortion, at least, to say that abortion is morally on a par with killing newborn babies is simply to say that abortion is morally impermissible.
Having accepted that abortion may be on a par with infanticide and that, if so, this would lead many to consider abortion unjustifiable, Boonin chooses to simply ignore this problem without providing any defence or argument:
A number of philosophers, including such prominent figures as Peter Singer and Michael Tooley, have argued that human infants do not have a right to life. And these arguments deserve to be taken seriously on their own terms. But they need not be taken seriously here. For the purposes of this book, arguments for the claim that human infants do not have a right to life can simply be set aside.
Whereas Singer and Tooley at least accepted the logical consequences of their pro-abortion arguments openly, Boonin is less honest. He chooses to "set aside" those logical consequences of his argument that he knows many will find horrific.
The Inconsistency of Pro-Abortion Libertarians
A few oddballs calling themselves libertarians have openly argued for both a pro- abortion and pro-infanticide position. In his 1895 article L'Enfant Terrible, Benjamin Tucker accepted that if one considers abortion justified then one also considers killing a newborn infant justified:
If, then, the child is the mother's while in the womb, by what consideration does the title to it become vested in another than the mother on its emergence from the womb and pending the day of its emancipation? I think that no valid consideration can be shown; and if such is the case, then it is established that the unemancipated child is the property of its mother, of which, by an obvious corollary, she may dispose as freely as she may dispose of any other property belonging to her.
In a 2019 article Abortion and Infanticide a Triple Libertarian and Critical-Rationalist Defence, Jan Lester argues for "the moral permissibility of the abortion and infanticide of unwanted humans" and states that "abortion and infanticide are, in themselves, morally neutral". Although the mind boggles at how anyone can evince such a profound level of moral bankrupcy, he is nonetheless correct when he states that the two positions are inseparable:
It is common, however, for people to accept some versions of these arguments as applied to abortion but reject them as regards infanticide. And that is simply to be logically inconsistent.
Yet most libertarian defenders of abortion want to hold the untenable position that abortion is valid but infanticide is not. A common way of attempting to square this circle is to argue that a baby in utero is some kind of aggressor by way of trespass, but this position relies on an unjustifiable denial of causal parental responsibility.
Libertarians who have attempted justifications of this kind include Murray Rothbard, Williamson Evers, and Walter Block. Rothbard's determination to deny causal parental obligations led him to argue the odd position that infanticide is justified if by deliberate starvation but not if by physical assault.
Most other pro-abortion libertarians fail to grapple with the logical implication of their view when it comes to infanticide. Unlike Benjamin Tucker and Jan Lester who say the quiet part out loud– openly advocating infanticide– pro-abortion libertarians usually either ignore their own inconsistency by not talking about infanticide or declare arbitrarily that they do not support it (despite supporting abortion).
If you are pro-abortion, at least have the honesty to accept the logical consequence: all your arguments for abortion also justify infanticide.