The term "positive eugenics" is an oxymoron. Anyone who thinks that humans could possibly manage reproduction and select offspring in a way that will be good for society as a whole should look at any thoroughbred population in the animal world. Thoroughbred race horses, German Shepherd dogs, Persian cats all of them pay terrible biological prices for the right to look just the way we want them to. The "blood" lines are documented, and the mating is controlled. The desirable offspring thrive and mate, the undesirables are sterilized or even executed.
It is the definition of "desirable offspring" that is the problem. When offspring display traits we can see that are desirable in our eyes, we want more animals with the same traits. The same thing happens with plants. The country of Holland went through a period of international intrigue and local banditry over tulips during the 17th century. It was all about selecting plants with appealing visual traits. Tomato eugenics have produced plants that bear fruits that survive shipping and handling and make it possible to enjoy tomatoes year-round.
What is downplayed when praising the "desirable offspring" of any eugenic management program is the common discovery that one or more undesirable traits must be tolerated in the plant or animal in order to get the desirable offspring. Thoroughbred race horses are fragile creatures that often break their bones when doing the one thing they are bred to do racing. German Shepherd dogs suffer painful and debilitating hip problems. Persian cats may have difficulty breathing. Tomatoes that survive a lot of handling often have little or no flavor, a sad side effect for a food item.
These consequences result because human beings are not smart enough to manage any genome human or otherwise. We can select for the desirable traits, but we can't make the offspring flawless and perfect. If we tried to manage human reproduction the way we manage race horses, we would almost certainly encounter one or several problems we could not avoid.
What if parents could, in fact, choose the desirable traits of their children, such as height, color of hair and eyes, intelligence, atheletic ability, and so forth. What if such choices inevitably led to a population that was 80% subject to pancreatic cancer by age 50? Would we want that? What if the consequence of eugenics in society led to such scorn for elderly, sick patients that all of them were routinely euthanized on some legal schedule? What if the practice of eugenics resulted in the passage of laws that authorized arrest and imprisonment for people who chose to dye their hair, or gain too much weight or wear contact lenses that changed their eye color? After all, would it be right for a person to destroy all that his parents had built in to his body by their careful selections before his birth?
And what would we do about kind, generous people with a tendency to cardiac disease? Would we forbid them to reproduce? Would we imprison or euthanize them because they were undesirable?
There is no way that any eugenics plan can produce an endless line of desirable offspring with no undesirable traits. The suggestion that human reproduction be managed the way we manage the reproduction of plants and animals is rendered ridiculous by the negatives embedded in the positive eugenics of horticulture and animal husbandry. Positive eugenics will always have negative issues as natural consequences. Society needs to focus on successfully rearing the offspring we are blessed with by natural processes. Humans should not embark on the practice of eugenics, because there is no way to have such a thing as "positive eugenics."