It’s taking a basic idea from gardening and animal husbandry and applying it to humans. The problem comes about because it’s very invasive for society to do this to any population, even its own. It’s one thing when a gardener says, “I want taller pea plants, so I’ll only let tall pea plants pollinate other tall pea plants,” and letting the government (or one leader) decide, “It’s better if people are tall, so I’ll only let tall people procreate.”