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

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 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.”