A number of smart people, Charles Murray included, are worried about "dysgenic pressure." The story, in brief, is that:
1. Intelligence is highly heritable.
2. The more intelligent have fewer kids than less intelligent.
3. Our average IQ is declining, or (here's the Flynn effect caveat) at least rising at a slower rate than it otherwise would.
4. We've got to get low IQ people to have fewer kids.
The response is predictable: People who find (4) offensive put their heads in the sand about (1), (2), and (3), and people who like (4) insist that it follows straight from the facts.
Once again, both sides are wrong. Yes, there is ample evidence that (1) and (2) are true. I've checked (2) using the NLSY myself, and found that the smartest people really do average almost one child less than people at the other end of the scale. And while all the data says that IQ is rising, it's hard to deny that IQ would have risen more if there were no relationship - or a positive relationship - between IQ and fertility.
But, by an argument parallel to my critique of eugenics using the Law of Comparative Advantage, (4) simply doesn't follow. What happens when low IQ people have more kids? It encourages greater specialization and trade. High-IQ people have a stronger incentive to focus on brainy work, because there are more low-IQ people to handle the non-brainy work.
Of course, another route to the same result would be for high-IQ people to have more kids. And it's plausible that if we could have one more person, it would be better for the world if he had a high IQ. But that's a trade-off we virtually never face in the modern world! There's plenty of food, and if low-IQ families have fewer kids, high-IQ families are not going to "take up the slack."
If you are really worried about dysgenic pressure retarding the advance of civilization, there are two sensible solutions. The first is to encourage high-IQ people to have more kids to increase the supply of brains; the second is to encourage low-IQ people to have more kids to increase the demand for brains. Urging either group to have fewer kids "for the good of society" is not smart.
P.S. If someone has zero or negative productivity in everything, the consolation of the Law of Comparative Advantage is admittedly hollow. But people this incompetent or malevolent are so rare that this counter-example is scarcely worth mentioning.