In 1832, Charles Knowlton, a doctor in Ashfield, Massachusetts, published a short book with a long title: “Fruits of Philosophy: The Private Companion of Young Married People, by a Physician.” Knowlton, who was thirty-one, was a “freethinker” and, by the standards of the Berkshires, an unusually adventurous soul. While attending the New Hampshire Medical Institute (now Dartmouth Medical School), he was too poor to pay for a dissecting class and so had liberated a corpse from a cemetery. For this, he was convicted of grave robbing and sentenced to sixty days in jail. In 1829, he wrote up his ideas about agnosticism in a tract and had a thousand copies printed at his own expense. Unable to find buyers in western Massachusetts, he took the copies to New York City, where he was arrested for peddling without a license.

In “Fruits of Philosophy,” Knowlton took up the subject of sex, or population growth, which, at the time, amounted to much the same thing. Like Thomas Malthus, whose work he cited, Knowlton was worried about the hazards of fertility. Using nineteenth-century birth rates, he projected that the number of people on the planet would double three times every century. Unlike Malthus, who saw no remedy except plague or abstinence, Knowlton believed that a more agreeable solution was at hand. What he called the “reproductive instinct” need not actually lead to reproduction. All that was required was some ingenuity. “Heaven has not only given us the capacity of greater enjoyment, but the talent of devising means to prevent the evils that are liable to arise therefrom; and it becomes us, ‘with thanksgiving, to make the most of them,’ “ he wrote.

Knowlton’s pamphlet provided his readers with easy-to-follow instructions. “Withdrawal immediately before emission” could, “if practiced with sufficient care,” be effective. A small piece of sponge, fitted with a narrow ribbon and inserted into a woman’s vagina “previous to connection,” would also suffice. If neither of these techniques appealed, he counselled “syringing the vagina immediately after connection, with a solution of sulphate of zinc, of alum, pearl-ash, or any salt that acts chemically on the semen.” As for the reliability of this last method, which he called the “chemical check,” Knowlton testified that he had discussed the matter with a gentleman who used to live in Baltimore, and that the gentleman had “no doubt of its efficacy.”

“Fruits of Philosophy” once again brought Knowlton into conflict with the law. Not long after the first edition appeared, he was charged with publishing obscene literature and fined fifty dollars. Even before the trial ended, he was indicted on new charges. This time, Knowlton was sentenced to three months of hard labor. In 1834, he was hauled into court for a third time. The third trial resulted in a hung jury, as did the retrial that followed.

But a good idea could not be kept down. Perhaps partly because of Knowlton’s legal trouble, “Fruits of Philosophy” was a popular hit. One of the jurors at the first trial told the doctor that, even though he’d seen no choice but to find him guilty, “still I like your book and you must let me have one of them.” In twenty years, the pamphlet—it was printed on tiny pages and could fit easily into a back pocket—went through nine editions in the United States. It was also published in Britain, where it sold roughly a thousand copies a year for nearly four decades.

Then, in 1877, two prominent British reformers, Annie Besant and Charles Bradlaugh, decided to reprint “Fruits of Philosophy” to test an obscenity statute. They were duly arrested, and their trial, at London’s Guildhall, became a national sensation. An estimated twenty thousand people gathered outside the hall each day to follow the proceedings. The pair were convicted, but this conviction was later thrown out on a technicality. Meanwhile, all the publicity surrounding the trial turned the once unmentionable business of birth control into a topic of daily discourse. By 1880, Britons had snatched up more than two hundred thousand copies of Knowlton’s tract.

“Fruits of Philosophy” has been credited, as much as any pamphlet can be, with changing the course of history. Right around the time it first appeared, fertility rates in the U.S. began to plummet, and, in the decades after Besant and Bradlaugh’s trial, British birth rates followed a similar trajectory. Though it can be difficult to tease out the causes of broad demographic trends, Knowlton’s work was instrumental in spreading what one historian has called “the good news that sex and procreation could be separated.” In other words, rather than being a consequence children became a choice.

What in Knowlton’s day was a decidedly imperfect choice is now an almost absolute one. Barring infertility or other complications—and despite the best efforts of Rush Limbaugh and Senate Republicans—couples today, at least in the U.S. and the rest of the developed world, can determine how many children they will have—five, four, three, two, one, or zero. Several recent books look at this decision from different vantage points, and come to surprising—some might say even alarming—conclusions.

In “Why Have Children?: The Ethical Debate” (M.I.T. Press), Christine Overall tries to subject that decision to morally rigorous analysis. Overall, who teaches philosophy at Queen’s University, in Ontario, dismisses the notion that childbearing is “natural” and therefore needs no justification. “There are many urges apparently arising from our biological nature that we nonetheless should choose not to act upon,” she observes. If we’re going to keep having kids, we ought to be able to come up with a reason.

Of course, people do give reasons for having children, and Overall takes them up one by one. Consider the claim that having a child benefits the child. This might seem self-evident. After all, a child deprived, through some Knowltonian means, of coming into existence, loses everything. She can never experience any of the pleasures life has to offer—eating ice cream, say, or riding a bike, or, for the more forward-thinking parents among us, having sex.

Overall rejects this argument on two grounds. First of all, nonexistent people have no moral standing. (There are an infinite number of nonexistent people out there, and you don’t notice them complaining, do you?) Second, once you accept that you should have a baby in order to increase the world’s total happiness, how do you know when to stop? Let’s say one kid eating ice cream represents x amount of added pleasure. In that case, two kids eating ice cream represents 2x, four kids 4x, and so on. The family with eight kids could perhaps afford to buy ice cream only half as often as the one with four. Still, provided the parents were able to throw in a bag of M&M’s, they (or, at least, the world) would fare better, total-happiness-wise, with the larger brood. And, from a strictly utilitarian perspective, things would be even better if the parents kept pumping kids out. Generalize this process, and the world would teem with more and more people leading less and less satisfying lives, until eventually the happiness of each individual would start to approach nil. This reductio ad Duggar Family was first articulated by the British philosopher Derek Parfit; it is known in academic circles as the Repugnant Conclusion. Overall considers it dispositive: “A simplistic utilitarianism is wrong about the ethics of having children.”

Overall finds most of the other frequently invoked rationales to be, philosophically speaking, similarly inadequate. Some people justify the decision to have children on the ground that they are perpetuating a family name or a genetic line. But “is anyone’s biological composition so valuable that it must be perpetuated?” Overall asks. Others say that it’s a citizen’s duty to society to provide for its continuation. Such an obligation, Overall objects, “would make women into procreative serfs.”

Still others argue that people ought to have children so there will be someone to care for them in their old age. “Anyone who has children for the sake of the supposed financial support they can provide,” Overall writes, is “probably deluded.”

Finally, lots of people offer the notion that parenthood will make them happy. Here the evidence is, sadly, against them. Research shows that people who have children are no more satisfied with their lives than people who don’t. If anything, the balance tips the other way: parents are less happy. In an instantly famous study, published in Science in 2004, the Nobel Prize-winning psychologist Daniel Kahneman asked nine hundred working women to assess their experiences during the preceding day. The women rated the time they’d spent taking care of their kids as less enjoyable than the time spent shopping, eating, exercising, watching TV, preparing food, and talking on the phone. One of the few activities these women found less enjoyable than caring for their children was doing housework, which is to say cleaning up after them.

But none of this really matters. Procreation for the sake of the parents is ethically unacceptable. “To have a child in order to benefit oneself is a moral error,” Overall writes.

David Benatar, a professor at the University of Cape Town, also turns to philosophy to determine the ideal family size. He gives away his answer in the title of his book, “Better Never to Have Been: The Harm of Coming Into Existence” (Oxford). The volume is dedicated to his parents, “even though they brought me into existence,” and to his brothers, “each of whose existence, although a harm to him, is a great benefit to the rest of us.” (It’s fun to imagine what family reunions with the Benatars are like.)

Benatar’s case rests on a critical but, in his view, unappreciated asymmetry. Consider two couples, the A’s and the B’s. The A’s are young, healthy, and rich. If they had children, they could give them the best of everything—schools, clothes, electronic gaming devices. Even so, we would not say that the A’s have a moral obligation to reproduce.

The B’s are just as young and rich. But both have a genetic disease, and, were they to have a child together, that child would suffer terribly. We would say, using Benatar’s logic, that the B’s have an ethical obligation not to procreate.

The case of the A’s and the B’s shows that we regard pleasure and pain differently. Pleasure missed out on by the nonexistent doesn’t count as a harm. Yet suffering avoided counts as a good, even when the recipient is a nonexistent one.

And what holds for the A’s and the B’s is basically true for everyone. Even the best of all possible lives consists of a mixture of pleasure and pain. Had the pleasure been forgone—that is, had the life never been created—no one would have been the worse for it. But the world is worse off because of the suffering brought needlessly into it.

“One of the implications of my argument is that a life filled with good and containing only the most minute quantity of bad—a life of utter bliss adulterated only by the pain of a single pin-prick—is worse than no life at all,” Benatar writes.

He acknowledges that many readers will have difficulty accepting such a “deeply unsettling claim.” They will say that they consider their own existence to be a blessing, and that the same goes for their children’s. But they’re only kidding themselves. And no wonder. Everyone alive today is descended from a long line of people who did reproduce themselves. Evolution thus favors a kind of genetically encoded Pollyannaism. “Those with reproduction-enhancing beliefs are more likely to breed and pass on whatever attributes incline one to such beliefs,” Benatar notes.

Taken seriously, Benatar’s logic leads to what might be called the Conclusive Conclusion. If we all saw the harm we were doing by having children and put a stop to it, within a century or so the world’s population would drop to zero. For Benatar, this is an outcome devoutly to be wished. “Humans have the unfortunate distinction of being the most destructive and harmful species on earth,” he writes. “The amount of suffering in the world could be radically reduced if there were no more” of us. Cultures more attuned to the tragic dimensions of existence glimpsed this truth long ago. Benatar’s title refers to the passage in Sophocles’ “Oedipus at Colonus” in which the chorus observes:

Never to have been born is best,

But once you’ve entered this world,

Return as quickly as possible to the place you came from.

It also alludes to an old Jewish saying: “Life is so terrible, it would have been better not to have been born. Who is so lucky? Not one in a hundred thousand.”

Like Overall and Benatar, Bryan Caplan believes that people need to think more rigorously about the decision to have children. And Caplan, too, draws on an academic discipline—economics, in his case—to provide some clarity. The result is “Selfish Reasons to Have More Kids: Why Being a Great Parent Is Less Work and More Fun Than You Think” (Basic).

According to Caplan, a professor at George Mason University, the major mistake that parents (or prospective parents) make is overvaluing the present. This is a common enough error. Workers in their twenties and thirties don’t save enough money for retirement because it seems such a long way off. Then their sixties roll around, and they wish they’d spent less on S.U.V.s and HDTVs and put more into their 401(k)s.

Couples, he argues, need to think not just about how many children they might want now, when they have better things to do than microwave Similac, but how many they will want to have around when they’re old and lonely and watching “The View.” Caplan recommends what he calls the “take the average” rule:

Suppose you’re thirty. Selfishly speaking, you conclude that the most pleasant number of children to have during your thirties is one. During your forties, your optimal number of kids will rise to two—you’ll have more free time as your kids assert their independence. By the time you’re in your fifties, all your kids will be busy with their own lives. At this stage, wouldn’t it be nice to have four kids who periodically drop by? Finally, once you pass sixty and prepare to retire, you’ll have ample free time to spend with your grandchildren. Five kids would be a good insurance policy against grandchildlessness.

Caplan does the math and concludes that in this case “the best number of kids is three.”

Although the figure may vary from one family to another, the same calculation, Caplan argues, applies across the board. Kids are a pain in the ass when they’re small. They require lots of care just at the time their parents tend to be busiest establishing themselves in their careers. As a result, most people stop producing children before they’ve reached the number that would, over the long haul, maximize their self-interest. “Typical parental feelings paired with high foresight imply more kids than typical parental feelings paired with moderate foresight,” Caplan writes. (Unfortunately, he does not explain what parents should do if their ideal number of children includes a fraction.)

Caplan concedes that some may feel compunction about having more (or any) children when they are already short on time and resources. Wouldn’t it be better to provide one or two children with a decent upbringing than to give three or four a lousy one? Here the good news, according to Caplan, is: it doesn’t matter. He cites a variety of twin and adoption studies showing that genetics swamps parenting on traits ranging from children’s health and intelligence to their chances of going to prison. There’s no need to monitor a kid’s French-fries consumption, or ferry him to music lessons, or teach him to avoid felony charges. As long as you “don’t lock him in a closet,” he’ll be O.K. Or not, as the case may be.

Parents who realize just how little difference hard work makes will work less hard. This should, in effect, drive down the cost of procreation and, by the logic of the marketplace, increase its appeal. “If kids are the product, consumer logic still applies: Buy more as the deal gets sweeter,” Caplan writes. At the very least, the additional kids will provide the world with more consumers and more labor: “Many think there’s no place for unskilled workers in the high-tech economy of the future, but someone has to do their jobs.”

Benatar’s child-rearing advice, if followed, would result in human extinction. Caplan’s leads in the opposite direction: toward a never-ending population boom. He declares this to be one of his scheme’s advantages: “More people mean more ideas, the fuel of progress.” In a work that’s full of upbeat pronouncements, this is probably his most optimistic, or, if you prefer, outrageous claim.

Back in “Fruits of Philosophy,” Knowlton used grade-school math to demonstrate the problem with sustained population growth. He imagined what would happen to the number of people on earth—at that point, around a billion—if it continued to increase “without check.” By the nineteen-thirties, it would reach eight billion. A century later, it would reach sixty-four billion.

Global population is expected to hit eight billion around 2025, which is to say about ninety-five years later than Knowlton predicted. No one in his right mind supposes that it could reach sixty-four billion without horrific consequences, except perhaps a few economists.

The decision to have a child, or one more child, or yet another child may seem to be a personal one—a choice about how many diapers you want to change in the short term versus how many Mother’s Day cards you hope to receive later on. But to see it in these terms alone is to be, as Caplan points out on the cover of his book, selfish. Whatever you may think of Overall’s and Benatar’s conclusions, it’s hard to argue with their insistence that the decision to have a child is an ethical one. When we set the size of our families, we are, each in our own small way, determining how the world of the future will look. And we’re doing this not just for ourselves and our own children; we’re doing it for everyone else’s children, too. ♦