Every movement needs a meliorist. In Ronald Green, an ethicist at Dartmouth College, proponents of genetic engineering have found an eager one. “We are entering the era of directed human evolution,” Mr. Green writes in “Babies by Design” (Yale University Press, 288 pages, $26). “Having vastly expanded our control over the world around us, our species is now rapidly developing the ability to alter the world within. The question is not whether we will do this, but when and how.”

Mr. Green gives readers a sampling of the science that will make this world possible, but he devotes most of his attention to answering the ethical questions these techniques raise. He is particularly concerned with parsing — and overturning — the distinction between what is therapy and what is enhancement. After positing that “gene doping” in sports is in our immediate future, for example, he writes, “This raises the question of why we would ban gene doping or prenatal genetic enhancement when we celebrate nature’s unequal bestowal of athletic talent.” He evinces a similar enthusiasm for extending the human life span, noting that as we learn more about the genetic basis of aging, “an extended life span may be one of the items on a future menu of prenatal choices.”

For Mr. Green, as for other enthusiasts of our new genetic powers, the increasingly blurry line between therapy and enhancement is not cause for concern, but for celebration, since it increases our “menu” of prenatal control over our children. Mr. Green cites the popularity of plastic surgery — originally developed as a therapy, now widely used as an enhancement — as evidence of our ability to accept enhancement. But the fact that plastic surgery has been democratized does not mean this change has been largely positive for society. And unlike genetic modifications, the results of cosmetic surgery are not passed on to one’s children, however much some taut-faced matrons might wish it were so.

Mr. Green is optimistic about a future where parents can use techniques such as preimplantation genetic diagnosis not only to avoid giving birth to children with diseases, but also to select for traits they value, such as athletic ability or proficiency in mathematics. To those who worry that such control might lead to a commodified, instrumental view of childrearing, Mr. Green offers what he calls the PLAAP principle: “Parental Love Almost Always Prevails” — “This principle sets forth the common truth that most parents end up loving whatever child they receive, no matter how much he or she conforms to — or disappoints — their prebirth expectations.” In an ethical feint common among enthusiasts of genetic engineering, Mr. Green adds, “overbearing parents are not confined to the world of prenatal genetic interventions. Parents already bring considerable pressure to bear on their children to develop talents or hone skills that the parents cherish or perceive in their offspring.”

But there are serious problems with this viewpoint. The choices made by overbearing parents today are not genetically permanent, and are made for their children alone, not for their grandchildren and great-grandchildren. Mr. Green suggests that, since genetic manipulation will undoubtedly lead to the creation of children who better meet their parents’ particular needs and expectations, families will experience less strife. “By increasing the likelihood that a child may actually accomplish what the parent wants,” he writes, “gene interventions could reduce conflict between parent and child.” This is not only an instrumental view of parenthood, but also an unusually narcissistic one.

“I expect that as we move into the era of human gene modification, parental love will continue to flourish,” Mr. Green writes. Well, yes. But this love will exist, as it does today, within a broader cultural context — one that will be radically transformed by the changes Mr. Green so eagerly encourages. Individual parents will surely continue to love their children, but it is not difficult to imagine a time when parents who fail to prevent the birth of a disabled child will be viewed not merely as unlucky, but as actively irresponsible. And that social censure will undoubtedly have an influence on the relationship those parents have with their child.

As for critics of the new genetic technologies, Mr. Green sees them as irrationally fearful of change — unfortunate victims of “status quo bias.” “Social science research has repeatedly shown that human beings resist change, even when there is no good reason to do so,” he notes.

Yet Mr. Green’s focus on this supposedly rampant “status quo bias” leads to a bias of his own: Like many ethicists who want to encourage genetic enhancement, Mr. Green is eager to distance himself from the past, most notably the eugenics movement of the early 20th century, with its compulsory sterilization laws and questionable science. He argues that the genetic modification he supports isn’t eugenics because it won’t be enforced by the state, won’t be based on shoddy science, and won’t be driven by elitist motivations. But that “special combination” of factors, as Mr. Green calls them, were, at the time, wholly acceptable mainstream views — at least among the well educated — and shouldn’t strike us now as social phenomena anomalous enough to discard.

Among other modest protections, Mr. Green proposes that parents’ choices about genetic modifications be limited to “accord with that of most other informed members of society.” But it was exactly that informed segment of society that once sterilized the unfit. How quickly the views of the meliorist can look like those of the mountebank! Among those who urge caution about genetic enhancements, it is not status quo bias, but a more cautious, tragic view of human nature, born of historical experience, that leaves them unsettled about the idea of parents selecting their children’s genetic traits.

Mr. Green’s argument, by contrast, suffers from the technocrat’s wild optimism, most notably the oft-repeated reassurance that, unlike previous eras, “we” will smooth out the ethical and scientific wrinkles of genetic enhancement. But who is this “we”? Who are these mandarins who will decide our collective genetic fate? One can only assume that they are people such as Mr. Green himself — and yet Mr. Green’s relentless enthusiasm for genetic engineering, and his impatience with the concerns raised about its dangers, leave the reader feeling like he is less a sober guide than a pushy stage mother, urging us to get out there and improve our future children! What, he seems to be asking, are we afraid of?

After all, as Mr. Green writes, “what nature accomplished in the past by means of natural selection, we may do by direction. Emerging genetic technology permits us to replace the destructive and wasteful process of natural selection with intelligence and design.” Our new genetic powers might permit us to do this and much more. But whether you welcome this change or fear it, it represents a radical reframing of human capabilities and human desires. “Genetically modified children — babies by design — are ends in themselves,” Mr. Green reassures us. The fact that he feels the need to remind us of this — that children are not merely means to our own ends, no matter how well-intentioned those ends might be — is a testament to just how far we have already traveled down the road of life-by-design.

Ms. Rosen is a fellow at the Ethics & Public Policy Center in Washington, D.C. and senior editor of The New Atlantis: A Journal of Technology & Society.