Catholic fertility is very low compared to the other groups listed. Whatever secret sauce for resisting modernity is built into evangelicalism, mormonism, etc, appears to be absent from high church denominations. Except for the hardcore traditionalists. We can expect to see more of them, as the others are bred out of existence.
> Except for the hardcore traditionalists. We can expect to see more of them, as the others are bred out of existence.
I would say conservative Catholics rather than just traditionalists.
Both SSPX and Opus Dei have larger families, and both have a strong conservative streak. But SSPX is traditionalist, whereas Opus Dei is more "modern conservative". Opus Dei is officially enthusiastic about Vatican II (provided it is "interpreted correctly"), SSPX refuses to accept it. While both celebrate Mass in Latin, SSPX insists on using the old (pre-1970) Tridentine Mass in Latin, whereas Opus Dei's Latin Masses are the contemporary Mass in Latin. Opus Dei emphasises obedience to the Pope (although Francis makes that painful in a way that his predecessors never did), SSPX insists on a duty to disobey papal decisions it believes to be erroneous