I am feeling too stupid to write a proper blog post (Spring doldrums). So I thought I would take the opportunity to make lemonade and write a short post about my own stupidity.
Every educated person should understand at least the very basics of Darwin. This is something very basic that I did not understand until a year or so back. Maybe some of you don't understand it either?
Let me pose it as simply and clearly as I can, as a multiple choice question:
There is a population of critters whose average (adult) height is 100. The height of any individual is determined partly by genetics and partly by environment, and different individuals have different heights because of different genes and different environments. But the population's genes and environment are not changing over time, so the average height stays at 100.
Suppose you take a (large) subset of that population, and isolate them from the main population in an environment that is the same as the original environment, and that also does not change over time. But you choose individuals that are taller than average, so the average height of your subset is 110. That first generation has children, and that second generation has an average height of 105. That is an illustration of regression toward the mean.
Question: if the second generation has children, what would you expect to be the average height of that third generation?
The correct answer is A: 105. Regression toward the mean stops at the second generation. If it continued for all subsequent generations, so average height eventually approaches 100, Darwin would make no sense, and it wouldn't be possible to breed cows for higher milk yields.
Somebody else can explain it better than me. Wikipedia is a good start.
This point had been vaguely puzzling me since I was a teenager. I didn't figure it out until I read one of Razib Khan's posts (I can't find the exact post). It cropped up again in this talk by Greg Cochran to a group of economists, and as you will see from the comments, I am not the only economist who was unclear on that point.
I wonder how many other supposedly educated people are like me?