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.

The size of your family helps determine how the world of the future will look. Illustration by Jim Stoten

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 2_x_, four kids 4_x_, 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: