I don't think I'm taking a bold stance by saying that any real attempt at eugenics is indefensible. Practically speaking, though, eugenics is just as much of a bust as it is morally. We can't positively select for "better people," and we may face dire consequences if we try to weed out genetic problems, too.
"Should" is a rather vague English word. Saying we "shouldn't" do something can mean that it is immoral to do it or that it won't have the desired result. When it comes to eugenics, we tend to circle around the first kind of "shouldn't," without paying attention to the second. Eugenics programs of the past have lead to attempted genocide, mass sterilization, and garden variety needless suffering. There are plenty of reasons for people to cut off the conversation about eugenics at the moral. Too often, though, that leaves the practical drawbacks unexamined. Beyond the possibility of bungling the job, there are concrete reasons why eugenics just wouldn't work.
Not All Animals Respond to Eugenics The Same Way
One of the primary arguments for practical eugenics resides right in a number of people's homes. There's no denying the effects of selective breeding on a species. Over only a few thousand years, humans have managed to breed in, or out, any number of traits when it comes to dogs. Certain breeds are smarter, stronger, faster, or bigger. It's proof of concept that selective eugenics can produce desired results.
But humans aren't dogs. In fact, almost no animal is a dog when it comes to its genetic plasticity. Everything about them makes them more practically suited to selective breeding. In canine DNA are specific sequences known as SINEC_Cf elements. These are sequences of DNA that are especially prone to wandering, whole from one part of the DNA strand to another. In dogs, they often insert themselves into stretches that act as regulatory agents on processes, vastly but for the most part safely changing the expression of genes. Dogs have about 11,000 of these sequences, and they go back to their wolf ancestors. Humans have less than one thousand. Dog DNA is also shown to have strange repeating segments more often than humans. They're prone to benign mutation. They're also, unlike many animals, prone to develop in a way that allows for more biodiversity. The skull of a puppy often doesn't much resemble the dog it will grow into, whereas other animals have juvenile forms that are more templates of their adult selves. Starting from a basic pattern and drawing on variation from there developmentally allows dogs a huge plasticity of form that cannot be copied by other animals, humans included.
Selective eugenics cannot do otherwise but have an effect. Obsessively manage a familial line over generations, and it will change a species. However, every species will respond differently. Assuming that eugenics will have as much of an effect on humans as it does on other species is wrong. Assuming it will have the same effect it does on the more genetically pliable species can be fatal.
It Will Squander Our Limited Biodiversity
Even the success that we see with dogs comes at a price. Though different breeds of dogs might have a trait that's desirable to humans, they aren't more fit to survive than their wild compatriots. And what becomes of all their selective breeding? Aside from any number of diseases, weaknesses, and health problems endemic to dog breeds, they lose biodiversity. It's estimated that five percent of wolves' diversity was lost when they became domesticated dogs. When those domestic dogs were obsessively bred to make, say, a golden retriever, they lost another thirty-five percent of their diversity.
Humans don't have that much biodiversity to lose. Grab any two humans on Earth and they're likely to be more similar to each other, at the genetic level, than two chimps from the same tribe. It's thought that the human race came close to extinction in the past, and that the few survivors became genetically close to each other. Losing another thirty-five percent of our diversity is not a tempting prospect. Going back to the dog model, scientists generally agree that their mutations don't involve introduction of new genes, but expressions of ones already existing ones, which is why they can still interbreed so well. All that difference in genetics is what allowed them to change form in order to adapt to different conditions. Human eugenics isn't going to be about trying to create many different breeds, but about going for an ideal. Limiting our biodiversity in the name of one ideal, or even a chosen few, doesn't just change the human species in the present, it cuts off our capacity for change in the future. It's widely acknowledged that a species that limits its gene pool leaves itself extremely vulnerable to any change from its ideal conditions. If the world itself changes — which is pretty much a guarantee — the human population could very well be stranded at a dead end.
It's No Way to Add Quality
So what would we gain for this vulnerability, and this expenditure of energy and care on selective human eugenics? What's the ideal trait that we'd like for future humans to have? The general consensus on what we'd like to breed into the human population is intelligence. The human brain wants to preen and protect itself. This separates us from the animals! Except there's no pure way, genetically, to do that. During a recent interview with Io9, Gary Karpen, a UC Berkeley biologist, has said flat-out that, given all possible genetic information about a child, it is in no way possible to predict intelligence. There are too many traits bound together, too many ways that genes might be expressed. The leader of the Human Genome Process, Francis Collins, said the same in his own book, claiming that no amount of genetic tinkering could give people designer babies with intelligence to order.
Well, what about other things? Strength? Fertility? Resilience? The problem is there is no one smart gene, or fertile gene, or strong gene. Mix the DNA of two geniuses and, even assuming somewhere in the soup of their DNA intelligence is passed down, it drags a net of other traits along with it. Those who manage animal breeding notice the same. When one can breed in a trait like swiftness in horses, or health and fertility in chickens, it generally comes with any number of other characteristics. Thoroughbreds and "hot-blooded" horses are notoriously temperamental. One study in poultry husbandry showed that even moderate increases in hen fertility and health came with increase in aggression, hysterical behavior, weird imprinting responses in the young, and odd sexual behavior. Good luck with that mixed in to the human population. Eugenics can't be a scalpel. It's a club. Even assuming we could get an extraordinary trait in one area, it would come with a whole host of other traits that wouldn't be so desirable.
We Can't Be Sure What's Unnecessary
But surely eugenics, at its most benign, can be used to eliminate the more terrible genetic conditions in a population? No one could actually approve of a human child suffering? Since the beginning of the concept, this was the most socially acceptable side of eugenics. Some would say that it's in practice today, since parents often consult genetic scientists to see if a fetus actually carries an incurable genetic disease.
At the same time, there are relatively few diseases that are guaranteed by a person's genes, and science is for the most part at work to use gene therapy to eliminate the expressed disease in one person, not the genes in an entire group. Even the elimination of genes that cause genetic disorders can pose a problem. For a some time, it seemed that genetically heritable diseases such as Tay Sachs and sickle cell anemia were biological mysteries. Why would any population pass down a disease that would kill the next generation? Recently, though, it was found that the sickle cell anemia allele helped protect against malaria, arguably the most widespread killer of humans, and that the allele that caused Tay Sachs protected against tuberculosis, another famous killer. Again, a complete elimination of these genes from the human population may very well lose humankind the capacity to naturally guard against two widespread non-heritable diseases.
Too often eugenics is dismissed on moral grounds by people who say science shouldn't meddle with nature because it's wrong, or cruel. Practically speaking, that's not the only "should" to use. To express the argument fully, we need to employ the practical use of the word "should." Selecting and deselecting certain genes of people shouldn't be done because it's the wrong way to go about building up a strong, healthy, and smart population. Instead, the widest array of genes should be supported. Differing genetics should be seen as opportunities to understand the hidden strengths of different human beings. And we should understand the many ways that stubborn genetic diversity benefits our society.