Republican presidential hopeful Ben Carson made headlines recently for comparing abortion to slavery, and women who have abortions to slaveowners.

On October 25, Carson, a retired neurosurgeon, said this to Chuck Todd on Meet the Press:

During slavery — and I know that's one of those words you're not supposed to say, but I'm saying it — during slavery, a lot of the slave owners thought that they had the right to do whatever they wanted to that slave. Anything that they chose to do. And, you know, what if the abolitionist had said, you know, 'I don't believe in slavery. I think it's wrong. But you guys do whatever you want to do'? Where would we be?

Many observers found Carson's remarks on abortion outrageous. Some had trouble understanding how Carson could be leading in the polls after saying something so shocking.

But this is not the first time Carson has compared abortion to slavery, nor is he the first conservative to draw the analogy. This idea — that fighting against abortion rights is similar to the abolitionist cause — is an incredibly common one in the pro-life movement.

"This kind of rhetoric is actually quite widespread in the movement, and it has been for some time," says Ziad Munson, a sociologist at Lehigh University who studies the pro-life movement. "I haven’t found a period in the movement where that kind of rhetoric was not being used."

Anti-abortion advocates compare abortion to slavery all the time, and Republicans often follow suit

David French argues in the National Review that Carson articulated "an entirely mainstream, pro-life view" that "most Republican politicians dare not utter." That is, that abortion is just as abhorrent as slavery, because both treat an entire class of people (black people in one case; in the other, "the unborn") as less than human.

"In both instances," French writes, "the abhorrent practice rested on dehumanization: the declaration that black Africans were somehow innately inferior to whites, and the declaration that unborn children are somehow less than fully and completely human."

The pro-life movement sees itself as championing the cause of social justice — as fighting for the rights of a voiceless, marginalized group of people. So they often appropriate the rhetoric of civil rights and related causes. Carol Tobias, president of the National Right to Life Committee, has compared the 40-year struggle of the pro-life movement to that of William Wilberforce, who spoke out against slavery in Britain for 40 years before the practice was finally banned. In a 2010 op-ed, Charmaine Yoest, president of Americans United for Life, invoked both the abolition of slavery and the civil rights movement.

Yoest also mentioned the Supreme Court's 1857 Dred Scott decision, which basically upheld slavery. Dred Scott is a popular comparison among anti-abortion advocates; they say that it's just as unjust a ruling as Roe v. Wade, and they hope that someday Roe will also be overturned.

How presidential candidates end up talking about abortion and slavery

This kind of rhetoric tends to trickle up from the anti-abortion grassroots to more mainstream Republican candidates. George W. Bush once confused pundits by mentioning Dred Scott during a presidential debate, but abortion opponents understood the Roe v. Wade reference loud and clear. Mike Huckabee recently put his foot in his mouth by suggesting that Dred Scott is actually still the law of the land, but that we still ended slavery nevertheless — which may have been subconscious wishful thinking on Huckabee's part about how to subvert Roe.

Other Republicans, like Carson, go farther with the slavery comparison. It's one thing to say that abolishing abortion is a human rights issue just like abolishing slavery was. It's another to draw more explicit comparisons between abortion and slavery — because that inevitably means equating women with slaveholders.

Huckabee has been saying this kind of thing for years — how we still haven't learned slavery's lesson that it's wrong for "one person" to "own another human being and have full control even to the point of life and death over that other human being."

None of this is a particularly good look for Republicans, or for the pro-life movement.

Comparing abortion to slavery has some really awkward historical implications

Pro-choice advocates, of course, oppose comparing slavery to abortion. The real "dehumanization," they argue, is denying women the bodily autonomy that abortion rights allow. But more than that, they say, the comparison is wildly offensive because it ignores the brutal history of reproductive coercion in slavery.

"If you think about it, the claim that abortion is like slavery is exactly backwards," wrote Imani Gandy at RH Reality Check. "I’m not a fan of comparing anything to slavery that is not slavery, but I’m fairly certain that we can all agree that slaveowners systematically forced Black women to give birth."

Slaveowners often used enslaved women as breeding stock to produce new future slaves. Women were raped by their masters and forced to bear the resulting children. They had no rights to their own offspring, much less their own bodies — they were sold or beaten if they couldn't or wouldn't reproduce, and if they did have children there was no guarantee they'd ever get to raise them. For black women forced to live as slaves, home remedies for contraception or abortion became forms of self-defense and resistance, and assertions of personal autonomy when they otherwise had none.

Comparing abortion to slavery brushes aside that history — especially the history of rape, which Carson doesn't believe should be a reason for legal abortion. It also trivializes women's need for autonomy, especially black women's.

Carson and other abortion opponents "think it’s clever to compare abortion to slavery or the Holocaust ... yet they fail to look at what actually happened during those times," said reproductive justice advocate Renee Bracey Sherman, who is a biracial black woman. "That’s offensive to all of our ancestors, and erases the very real experiences of black women today."

Good idea or bad, this kind of thing just keeps on happening

If comparing women's health care to slavery reliably causes so much backlash, why do Republicans like Carson keep saying these things?

It could just be simple pandering to the Republican base. That's certainly a plausible explanation in Carson's case; he's seeking the Republican nomination and may be inclined to puff up his pro-life credentials as a result. Plus, Carson's own views on abortion may be less extreme than he lets on. Although he refused to perform abortions in his own medical practice, he would still refer women to other doctors for medically necessary abortions. But by his own Meet the Press argument, Carson would likely look down on the kind of "principled objection" to slavery that just referred a slaveowner to another town to make a purchase.

Still, there's probably more going on than cynical political ploys.

"It’s not always clear to me they think there’s anything wrong with it," Munson said. "It comes as a surprise to them. I don't think they are misspeaking — they are saying things they think are completely unproblematic."