When you look at pictures from the Arab spring, you see these gigantic crowds of young men, and it confirms the impression that the Muslim Middle East has a gigantic youth bulge — hundreds of millions of young people with little to do. But that view is becoming obsolete. As Nicholas Eberstadt and Apoorva Shah of the American Enterprise Institute point out, over the past three decades, the Arab world has undergone a little noticed demographic implosion. Arab adults are having many fewer kids.

Usually, high religious observance and low income go along with high birthrates. But, according to the United States Census Bureau, Iran now has a similar birth rate to New England — which is the least fertile region in the U.S.

The speed of the change is breathtaking. A woman in Oman today has 5.6 fewer babies than a woman in Oman 30 years ago. Morocco, Syria and Saudi Arabia have seen fertility-rate declines of nearly 60 percent, and in Iran it’s more than 70 percent. These are among the fastest declines in recorded history.