Several loyal Marginal Revolution readers ask for discussion of IQ:
"There is something special about IQ. We must conserve IQ at very high cost, and gains in IQ will bring very high social returns."
Actions to improve average IQ are eugenics, or else they are not meaningful. This conclusion does not require that IQ be entirely, or even majority, genetic in origin. Any measures to improve the non-genetic portion of IQ are ephemeral as they are not propagated across generations, and are also tantamount to expending resources teaching people to take IQ tests. [Cynics will argue that this is what education mainly consists of, but few will argue that it is the best goal for education.]
Here I am ignoring the idea that immigration should be restricted to "conserve IQ". Besides its repugnant nature, this is a very weak argument. You can reach the same conclusions on immigration by arguing for greater social assimilation, and against dilution of America's culture of honesty and opportunity; and these latter arguments have some justification outside mere prejudice.
I also oppose the conflation of IQ with social productivity. Oddly, the two are often merged by self-styled progressives, seeking to minimize the case that poverty is self-inflicted. I have written before about this.
Finally, there is a good case to be made that we should encourage reproduction among the upper classes, if only because demographic decline seems a greater threat than overpopulation. This can only be accomplished by ending the lifelong discrimination against mothers in skilled jobs, which is fantastically expensive in its own right. This spills into an unrelated point, which is that social welfare would be greatly increased by getting as many 20-25 year olds as possible out of college.
[Mr. Cowen's own answer is here.]