True. And nobody has a clue why. It seems to hold across most societies.
My own speculation is that improved pre-natal and childhood nutrition has an effect.
Also migration from rural to urban has a stimulating effect.
Whether those gains can be sustained over the long term is open to question.
1. A modern nation's contraception/abortion policies have a way of culling the ranks of its less intelligent cohorts.
2. The proportion of mentally retarded people in a modern society tends to diminish over time, due to medical advances and selective abortion.
Item #2 is an interesting one, for the results seem to be clear in some "high-intelligence" nations like Japan. I don't think I've ever seen a mentally retarded Asian in my life.