There's a belief on the American left that says it's impossible to be both a principled libertarian and a principled pro-lifer—that the two positions are somehow intellectually incompatible. It's been popping up more often lately as liberal writers look for ways to criticize Sen. Rand Paul, as in this Salon piece, where the author says Paul and his father "have always played fast and loose with their libertarian principles when it comes to reproductive health."

The unstated premise on which that statement relies is that No True Libertarian could also be against abortion. But in reality, it's not the case that all libertarians believe women should have the right to terminate a pregnancy. More to the point, it's flatly incorrect to suggest that opposition to legal abortion is irreconcilable with the belief system that places a person in the libertarian camp.

What is true is that most libertarians—at least historically—have held pro-choice views. In their 2012 book The Libertarian Vote, David Boaz, David Kirby, and (former Reason Foundation polling director) Emily Ekins looked at the data and confirmed as much. "According to our analysis of 2008 [American National Election Study] data, 62 percent of libertarians are pro-choice versus 37 percent pro-life, similar to percentages of the national population," they wrote. Stated otherwise, as recently as 2008, a six-in-ten majority of libertarians thought women should be able to legally get an abortion.

Here at Reason that tendency has been even more pronounced, with the magazine's editorial staff overwhelmingly favoring a right to legal abortion. You've probably seen my colleague Elizabeth Nolan Brown's writing on the subject, including her recent quasi-defense of Planned Parenthood. From the general tenor of our coverage of this issue over the years, one might get the impression that most if not all prominent libertarians support the so-called "right to choose."

Yes, libertarians tilt pro-choice. But as a young, female, pro-life Roman Catholic who also happens to identify as libertarian (and who works for a magazine dedicated to free minds and free markets), I'm always rather dismayed when someone on the far side of the political universe professes to be an authority on what "consistent libertarianism" requires.

From my perspective, the consistent libertarian position on abortion is contingent—it depends whether you believe the entity developing in the womb counts as a human being.

I accept that some people don't think it does. And if I were one of them, I'd probably be pro-choice too. Like many libertarians, a fundamental question I use to adjudicate whether an act should be considered a crime is whether or not it has a victim. Drug use? Consensual prostitution? Working for less than the wage some politician has decided should be the legislatively mandated minimum? I oppose government intervention to stop any of these things, because none of them involves the use of force by one human being against another.

But for the consistent libertarian who looks at an ultrasound and sees a baby, a person, a fully human life, it's extraordinarily hard to avoid the conclusion that abortion is an act of violence.

That's where I come down. No doubt my Catholic faith has something to do with it, but so does my (admittedly imperfect) understanding of the science of what happens during conception and at the various stages of fetal development. In moments of honest reflection, alone only with my conscience and my God, this is the inescapable conclusion I can't help but arrive at.

And while I may be in the minority among libertarians, I'm definitely not alone. In 1978 Reason published an entire issue dedicated to debating this topic. There, in a piece called "It's a Matter of Life and Death," the author Karl T. Pflock made the pro-life libertarian case as follows:

The unborn are human beings. Unborn human beings are persons. Unborn persons are innocent of aggressive behavior. Therefore, except in a "life-boat" situation, they must be accorded the protection of the nonaggression principle. And, therefore, under normal (nonemergency) circumstances, abortion is a violation of the cardinal principle of libertarianism.

Recall as well that the authors of The Libertarian Vote found more than a third of libertarians opposing abortion. There's even reason to suspect that number might be on the rise. Although millennials are routinely painted as socially liberal, a 2014 Pew Research study found that people 18 to 29 are actually more likely than those 30 to 64 to say the practice should be illegal. A majority of Hispanics, the fastest-growing demographic in the country, also said it should be banned.

For obvious reasons, the largest divide is between what Pew calls "consistent conservatives" (just 19 percent of whom said abortion should be legal in all or most cases) and what Pew calls "consistent liberals" (88 percent of whom answered the same). It's worth noting, though, that even on the political left, few people will go so far as to say abortion should always be allowed.

At a recent Young Americans for Liberty conference, The Washington Post's David Weigel remarked that attendees were largely unbothered by Paul's belief that life begins at conception. "I don't think a civilization can long endure that does not have respect for all human life, born and not yet born," the Kentucky senator said in 2012. Weigel wrote that many liberals see that stance "as antithetical to 'choice.' Doctrinal libertarians don't necessarily agree."

That shouldn't be surprising. The philosophy of liberty simply cannot speak to whether abortion is an act involving one human being or two. For that, you have to look elsewhere—at science or experience or religion or tradition or intuition or a combination of all those things. How you answer the question will determine your starting point. Libertarianism can only show you where to go from there.

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If you're interested in this subject, you may want to check out the great Reason abortion debate of 2013, featuring Managing Editor Katherine Mangu-Ward and Science Correspondent Ron Bailey arguing the pro-choice side and Mollie Hemingway (then of GetReligion and Ricochet, now of The Federalist) arguing the pro-life.