" It’s the bell curve again,” I said to Ishtar. “If—as Lazarus thinks, and statistics back him up—every migration comes primarily from the right-hand end of the normal-incidence curve of human ability, then this acts as a sorting device whereby the new planet will show a bell curve with a much higher intelligence norm than the population it came from…and the old planet will average almost imperceptibly stupider.”

“Imperceptible except for one thing!” Lazarus objected.

“That tiny fraction that hardly shows statistically is the brain. I recall a country that lost a key war by chasing out a mere half dozen geniuses. Most people can’t think, most of the remainder won’t think, the small fraction who do think mostly can’t do it very well. The extremely tiny fraction who think regularly, accurately, creatively, and without self-delusion - in the long run these are the only people who count and they are the very ones who migrate when it is physically possible to do so "

― Robert A. Heinlein