Once, pretty much everywhere, beating your wife and children was regarded as a father's duty, homosexuality was a hanging offense, and waterboarding was approved — in fact, invented — by the Catholic Church.
Through the middle of the 19th century, the United States and other nations in the Americas condoned plantation slavery. Many of our grandparents were born in states where women were forbidden to vote. And well into the 20th century, lynch mobs in this country stripped, tortured, hanged and burned human beings at picnics.
Looking back at such horrors, it is easy to ask: What were people thinking?
Yet, the chances are that our own descendants will ask the same question, with the same incomprehension, about some of our practices today.
Is there a way to guess which ones? After all, not every disputed institution or practice is destined to be discredited. And it can be hard to distinguish in real time between movements, such as abolition, that will come to represent moral common sense and those, such as prohibition, that will come to seem quaint or misguided. Recall the book-burners of Boston's old Watch and Ward Society or the organizations for the suppression of vice, with their crusades against claret, contraceptives and sexually candid novels.
Still, a look at the past suggests three signs that a particular practice is destined for future condemnation.
First, people have already heard the arguments against the practice. The case against slavery didn't emerge in a blinding moment of moral clarity, for instance; it had been around for centuries.
Second, defenders of the custom tend not to offer moral counterarguments but instead invoke tradition, human nature or necessity. (As in, "We've always had slaves, and how could we grow cotton without them?")
And third, supporters engage in what one might call strategic ignorance, avoiding truths that might force them to face the evils in which they're complicit. Those who ate the sugar or wore the cotton that the slaves grew simply didn't think about what made those goods possible. That's why abolitionists sought to direct attention toward the conditions of the Middle Passage, through detailed illustrations of slave ships and horrifying stories of the suffering below decks.
With these signs in mind, here are four contenders for future moral condemnation.
Our prison system
We already know that the massive waste of life in our prisons is morally troubling; those who defend the conditions of incarceration usually do so in nonmoral terms (citing costs or the administrative difficulty of reforms); and we're inclined to avert our eyes from the details. Check, check and check.
Roughly 3 percent of adults in this country are incarcerated. We have 4 percent of the world's population but 25 percent of its prisoners. No other nation has as large a proportion of its population in prison; even China's rate is less than half of ours. What's more, the majority of our prisoners are nonviolent offenders, many of them detained on drug charges. (Whether a country that was truly free would criminalize recreational drug use is a related question worth pondering.)
And the full extent of the punishment prisoners face isn't detailed in any judge's sentence. More than 100,000 inmates suffer sexual abuse, including rape, each year; some contract HIV as a result. Our country holds at least 25,000 prisoners in isolation in so-called supermax facilities, under conditions that many psychologists say amount to torture.
Industrial meat production
The arguments against the cruelty of factory farming have certainly been around a long time; it was Jeremy Bentham, in the 18th century, who observed that, when it comes to the treatment of animals, the key question is not whether animals can reason but whether they can suffer. People who eat factory-farmed bacon or chicken rarely offer a moral justification for what they're doing. Instead, they try not to think about it too much, shying away from stomach-turning stories about what goes on in our industrial abattoirs.
Of the more than 90 million cattle in our country, at least 10 million at any time are packed into feedlots, saved from the inevitable diseases of overcrowding only by regular doses of antibiotics, surrounded by piles of their own feces, their nostrils filled with the smell of their own urine. Picture it — and then imagine your grandchildren seeing that picture. In the European Union, many of the most inhumane conditions we allow are already illegal or — like the sow stalls into which pregnant pigs are often crammed in the United States — will be illegal soon.
Institutionalized, isolated elderly
Nearly 2 million of America's elderly are warehoused in nursing homes, out of sight and, to some extent, out of mind. Some 10,000 for-profit facilities have arisen across the country in recent decades to hold them. Other elderly Americans may live independently, but often they are isolated and cut off from their families. (The United States is not alone among advanced democracies in this. Consider the heat wave that hit France in 2003: While many families were enjoying their summer vacations, some 14,000 elderly parents and grandparents were left to perish in the stifling temperatures.) Is this what Western modernity amounts to — societies that feel no filial obligations to their inconvenient elders?
Sometimes we can learn from societies much poorer than ours. My English mother spent the last 50 years of her life in Ghana, where I grew up. In her final years, it was her good fortune not only to have the resources to stay at home, but also to live in a country where doing so was customary.
Keeping aging parents and their children closer is a challenge, particularly in a society where almost everybody has a job outside the home (if not across the country). Yet the three signs apply here as well: When we see old people who, despite many living relatives, suffer growing isolation, we know something is wrong. We scarcely try to defend the situation; when we can, we put it out of our minds. Self-interest, if nothing else, should make us hope that our descendants will have worked out a better way.
Of course, most transgenerational obligations run the other way — from parents to children — and of these the most obvious candidate for opprobrium is our wasteful attitude toward the planet's natural resources and ecology. Look at a satellite picture of Russia, and you'll see a vast expanse of parched wasteland where decades earlier was a lush and verdant landscape. That's the Republic of Kalmykia, home to what was recognized in the 1990s as Europe's first man-made desert. Desertification, which is primarily the result of destructive land management practices, threatens a third of the Earth's surface; tens of thousands of Chinese villages have been overrun by sand drifts in the past few decades.
It's not as though we're unaware of what we're doing to the planet: We know the harm done by deforestation, wetland destruction, pollution, overfishing, greenhouse gas emissions — the whole litany. Our descendants, who will inherit this devastated Earth, are unlikely to have the luxury of such recklessness. Chances are, they won't be able to avert their eyes, even if they want to.
Let's not stop there, though. We will all have our own suspicions about which practices will someday prompt people to ask, in dismay: What were they thinking?
Even when we don't have a good answer, we'll be better off for anticipating the question.
Kwame Anthony Appiah, a philosophy professor at Princeton University, is the author of "The Honor Code: How Moral Revolutions Happen."