Earlier quoted context omitted.
Bullshit. USSR did the same thing about the same time.
People stop having kids when economic forces work against them. It doesn't matter how the forces were put there. At least it's not like it was in Adam Smith's time, when the downward pressure was a result of children failing to reach adulthood. Thanks to birth control and modern medicine, people can decline to have children when they know the conditions are not right.
1. Decline in religious belief
2. Widespread, easy to use birth control
3. More options for entertainment are widely available. Having kids is less attractive when you could be traveling, binging endless series, or playing computer games
4. Strong anti-natalist sentiment that was pushed very hard post-war and persists
5. Higher rates of college attendance, which means people are studying and furthering their careers in their 20s rather than having kids