The baby boom is the best kind of sociological mystery.

Everybody knows what it is. Nobody knows why it happened.

That might sound like hyperbole. It’s not. There are practically no papers that offer a persuasive or confident explanation for why the 200-year decline in fertility was interrupted exactly once between the 1930s and 1960s. Almost every theory has major holes in it. Just an amazingly fun thing to think about — with major implications, if governments want to engineer a SECOND baby boom.