Salko Spandrell understood so much, and yet he couldn't understand regression to the mean.

Spandrell ?

Salko Hi! Big fan. I didn't realize you were back on twitter! Your old blog had a post suggesting that regression to the mean in intelligence is observed only because of “smart men marrying attractive women,” which is retarded. There's a fundamental RttM selection effect.

Spandrell Well “only” is an overstatement but the aggregate result isn't only genetic. If smart couples breed consistently over generations the average does go up. How else do you explain Indian castes?

Salko Wdym by the aggregate effect? If the heritable part of intelligence is a linear combination of the parents, you can't change the population mean by assortment. I think RttM is commonly understood as the tendency of extreme samples to come closer to the general mean when retested. In this case, the fact that children's intelligence tends to be closer to the population mean than the average of their parents.

I think the fungibility of traits is probably part of this, but the basic statistical effect behind RttM is that selecting for a high combination of deterministic and stochastic components means that extreme samples will tend to have extreme stochastic components (obviously). Only the deterministic component is preserved, though, so the next tests will give less extreme results.