I’m tempted to say, in response to Robert VerBruggen’s lament, that yes, that’s Twitter for you, and this is one of many reasons why I’m not on it. But I think there is more to say about the problem of eugenics than merely “it’s immoral but not ineffective.”

First of all, as I’m sure VerBruggen would agree, not all efforts to improve the gene pool are immoral, and though we may disagree about exactly where the line is, we both surely agree that it’s laudable to get tested for Tay-Sachs before you marry, and we both surely agree that forced sterilization of “undesirables” is an abomination. For myself, I’ve written about this before, and I stand by what I wrote then.

Second, we should probably limit the word “eugenics” to collective programs to improve the gene pool, and not apply the word to individual choices about who to have children with, because only collective programs can actually change the population as a whole. As such, it’s important to recognize that to breed for particular traits, you have to prevent elements within the population that don’t have those traits from breeding. For example, if you assume that intelligence is highly heritable, and wanted to increase the intelligence of the population, it wouldn’t do to get smart people to marry other smart people. You’d have to get smart people to outbreed less-smart people. I can’t think of a way to do this that is both ethical and plausible — and most of the ways I can think of are neither.

Finally, while we know from extensive experience in selectively breeding animals and plants that such programs work, by “work” we mean that we’ve maximized particular traits, abilities and behaviors. And in the course of doing so, you always get tradeoffs. The swift greyhound has chronic hip problems. The highly-trainable poodle is also prone to stress. The large-breasted chicken can’t fly. And so forth.

There is no reason to doubt that the same would be true of humans, and that any serious attempt to breed people for particular traits — even if undertaken on an entirely voluntary basis and involving no abortion or sterilization or whatnot — would have unexpected side effects. Perhaps breeding for ambition will result in lower empathy. Perhaps breeding for intelligence will result in greater incidence of anxiety and depression. Perhaps breeding for greater athletic prowess will result in higher rates of marital infidelity and divorce. Who knows?

We don’t — and we can’t ethically conduct the kinds of controlled experiments that would allow us to determine with high confidence that we had avoided unexpected side effects. That caution holds as well for genetic therapies that are surely on the horizon. “Fitness” is only meaningful relative to a set of environmental conditions. Narrow the set of traits by which you define fitness and you have implicitly narrowed the set of environments within which an organism will prove fit. Which is not, generally, a good way for a species to maximize its survival prospects.

I’m not arguing that people should blithely ignore genetic history or the science of inheritance more generally in matters like mate selection. (If I did, nobody would listen to me anyway.) But I am arguing both for humility and for a broad understanding of what constitutes fitness. Someone especially smart who says, “I need to marry someone just as smart as I am so that our children are likely to be similarly smart and hence similarly successful” is not only running the risk of disappointment due to mean-reversion (which remains a factor even when you stack the deck in your favor), but running the risk of having ignored other vital dimensions of the human personality by reducing “fitness” to a narrow, measurable trait.

(Also, if you want a good marriage, you should probably marry someone who you love and desire, who is good for you and who you are good for, and with whom you share certain core values and a robust ability to communicate, rather than thinking of your spouse primarily as breeding stock. Not to mention not treating your children as pint-sized success machines. And staying off Twitter when your wife is in the next room with the OB/GYN. Just saying.)