One way to interpret current sociological twin and adoption studies is to conclude that parents, for various reasons, have no real impact on how their children turn out as adults. The numbers point to one simple fact: identical twins separated at birth and interviewed years later show tremendously high correlation on everything from height to higher intelligence.

This priority of nature over nurture is the basis of Bryan Caplan’s argument in his latest book, Selfish Reasons to Have More Kids: Why Being a Great Parent Is Less Work and More Fun Than You Think. Of course, Caplan’s aim isn’t to exalt the unattractive features of this seemingly unlikely finding—after all, being outed as inconsequential wouldn’t motivate many to take on the burden of parenthood. Rather, Caplan, a George Mason economics professor, seeks to show that the costs of childrearing are grossly inflated, and that parents who think 40-plus hours of childcare per week is “normal” are deeply misguided.

The brilliance of Selfish Reasons to Have More Kids is the author’s track toward his final conclusion, namely, that we ought to “stock up” on children, not dismiss child-rearing because we over-estimate each child’s marginal cost to us. Caplan’s case is purely economic; its specific, clear focus makes his argument surprisingly strong, curiously interesting, and somehow compelling.

Of course, Caplan doesn’t try to imply that having more kids is always merrier. The numbers, in terms of both “happiness” and marginal benefits, stack up against this. In fact, “happiness and number of offspring move in opposite directions.” But that’s not the whole story. As Caplan cites, according to the General Social Survey, each child detracts only about one percentage point from overall happiness. And if you’re married, you’ve already got an 18 percentage point higher chance of being “very happy” than an unmarried counterpart. The net conclusion, says Caplan, is that if you’re “married with children, you’re far more likely to be happy than if you’re single and childless.”

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Interesting statistics, no doubt. Maybe an extra kid isn’t such a bad idea after all?

Before painting too cozy a picture, remember that Caplan’s arguments aren’t designed with some deeper “value” criterion in mind. Rather, they are utilitarian calculations that happen to align with what a traditional view of marriage and family reveres as the fruits of a well-ordered life. In the end, we have to conclude that if having more kids didn’t contribute to parents’ flourishing, the economic argument in favor of doing so wouldn’t hold any water.

To show that parental input has little to no lasting effect on a child’s tendencies and character as an adult, Caplan points to twin and adoption studies. Employing a “switched at birth scenario,” he repeatedly racks up statistical research against commonsense notions of adolescent formation. In almost all cases—from lifespan to religious values to criminal futures—the long-term effects of parental nurturing (assuming minimally decent parenting in a stable family) are equally unimpressive and, regrettably for many of us, undetectable. For example, the Minnesota Study of Twins Reared Apart found that, when given IQ tests, separated adult identical twins scored regularly within 6 percentage points of one another (regardless of where on the spectrum they fell), while non-biological siblings regularly saw as great as a 30 percentage point difference in scores. The same study also found that twins “raised apart were more alike in happiness than twins raised together. If you’re happier than four out of five people, expect your separated identical twin to be happier than 67 percent of us.” That’s a surprisingly high rate of similarity, especially for such a volatile thing as self-reported “happiness.”

Caplan cites too many studies to outline them all here, but suffice it to say that the evidence for a priority of nature over nurture—at least in terms of long-range consequences—is overwhelming.

Given these findings, Caplan’s ultimate conclusion—that it benefits us to have more kids rather than fewer—might strike us as counterintuitive. If all I can expect of my children is diminishing net happiness and unpredictable outcomes, why opt for more of them? But there’s a rub: since parenting minimally affects how kids turn out, Caplan’s citations imply that parental stress is largely superfluous. Therefore, time commitments to childcare can be eased with no probable deleterious effect on a child’s long-term wellbeing. And assuming that many of the benefits of children come later in life, such as welcoming grandchildren and personal geriatric care, the greater net value of having an extra kid or two now seems economically demonstrable in the long run.

So far as we can weigh benefits and losses in terms of costs and expenditures, Caplan’s case is a strong one. When faced with the trickier points of value judgments, however, his number-crunching methodology falters under its own burden. Not surprisingly, at most of the points where ethical or moral language would tend to arise—for example, dealing with discipline, education, or religion—the author’s clear libertarian sensibilities commit him to shoulder-shrugging. “I am simply observing,” he concedes, “that parents of all stripes want their kids to share their values and believe they can cause this to happen.”

Caplan’s decision to stay out of serious value confrontation is a predictable symptom of the modern social scientist’s perspective. Nevertheless, there are a number of points at which the reader would benefit greatly from further ethical reflection. It seems strange, after all, that any argument in favor of progeny could proceed without touching on something beyond the mere taxonomy of facts—in other words, without assessing the sorts of goods and non-goods that are often essential to the basic units of family planning, i.e., people.

Although Caplan admittedly aims to encourage some people merely to “rethink” family planning rather than to advocate unchecked proliferation, a glaring feature of Caplan’s case is the self-imposed breakdown of his own analytic process for determining whether or not “the birth of another baby [would] make the world better—or worse.” Needless to say, the basis of this question is purely utilitarian; but to form such a question requires at least some acquiescence in the idea that human life is inherently valuable. And this value judgment is something Caplan refuses to grant. (At best, his granting that human life is in fact valuable would ground the value of life in terms of economic optimality, not its own intrinsic goodness.)

Consequently, Caplan’s conclusions about the birth of new babies are somewhat muddled and confusing. To give an example:

At least one person benefits tremendously from virtually every birth—the new baby. The baby benefits if the parents have him out of loneliness, or to hold their marriage together. The baby benefits if the parents are poor, or workaholics, or carry Huntington’s disease in their genes. Turning the gift of life into a curse is quite a challenge.

Here and elsewhere, Caplan’s attempts to treat procreation as a moral good using a Pareto model of efficiency appear unprofitable. Sure enough, he identifies life as a “gift,” but applying terms like “benefit” and “better” to the possibility of creating new life is contrived. “Benefit” and “betterment” both presuppose some foundation in reality—some real goodness that can both be affected and affect; and even a “twinkle in your daddy’s eye” doesn’t amount to that. The operative assumption here, on Caplan’s part, is that utility itself is something real, and that it preexists each instance of individual life. Ethically, this assumption is a stretch that’s hard to make. And it leaves the door open to a host of similarly vague conclusions.

On the other hand, to argue that a new person “benefits tremendously” by being born presupposes that life is basically good, that it is better than the alternative, and that the world itself benefits from each life. Any calculation of utility depends on this foundational, natural goodness for its force. Furthermore, even if new life is somehow “objectively” beneficial only in an economic sense, such objectivity is flimsy insofar as it rests not on natural goodness (i.e., the goodness of being human), but instead on some artificial goodness of cost versus loss. In other words, economic goodness is a relational notion; and this conception of goodness isn’t absolutely “objective.” Thus, to say that life entails absolute “benefit” in (almost) all cases—and that life is a “gift”—is an attempt by Caplan to say that “life is naturally good.” But his aversion to value language makes this suggestion forever unclear.

Given the assumed limitations of Caplan’s viewpoint, his stance toward artificial forms of procreation for those wanting to enter the kid market with limited capital is hardly surprising. To be sure, the strongest arguments in the book revolve around the ability to assess risk and reward, and things all but disintegrate when he begins moralizing on the topic of “human dignity” vis-à-vis artificial procreation, dismissing those who morally reject in vitro fertilization (IVF) and pre-implantation genetic diagnosis (PGD) as “splitting hairs.” Because of his commitment to value impartiality—i.e., his unwillingness to base “benefit” and “loss” on real, natural goods—Caplan fails to see that the “cruelty” he perceives in spanking is as nothing compared with the embryonic reduction that’s part and parcel of pre-implantation selective IVF. “If you don’t want your children to have sickle-cell anemia,” he says, “keep affected embryos away from your womb.” So much for the “gift of life.”

Despite some serious shortcomings in the final analysis, Selfish Reasons to Have More Kids is a worthwhile read, if for no other reason than that it offers an accessible, convincing look at the reasons that many modern parenting practices are untenable. From the perspective of an economic theorist, Caplan appreciates the value of human life—so much so that he attempts to argue that another life (or two) is almost always an objective good for the family. But his moral prowess comes up short, and all his value judgments about natural goods are reduced to calculable costs in the marketplace of utility. It’s unfortunate that, for all the skill and innovation of Caplan’s initial arguments, he fails to realize that overextending economic calculus into the realm of moral philosophy does more to hurt than to help his case. But to separate this shortcoming from the book’s valuable undercurrents won’t be too difficult a task for the discerning reader.