Fertility rates are strongly correlated with population density. Europe is more densely populated than the US. Thus it currently has low fertility. Once the population of Europe drops, their fertility will increase and the population will level off. The United States’ population will increase until it gets more crowded, then it will drop.

I suspect that both religiosity and government footprint is also related to population density.

No gloom and doom. Only simple feedback mechanisms.

More info here: