Perhaps 's/liberalism/modern capitalism/g'? Outside of a few ultra-religious niches, family sizes for modern conservatives are also plunging. Today's all-noses-to-the-grindstones short term economic "maximization" is squeezing ~every voluntary but not-quickly-monetized human activity out of existence. And it has long since figured out that a couple having one kid - whose "normal" birth runs up $100,000+ in medical expenses, and whose college education will cost $500,000+ - is a far better outcome for Wall Street than anyone being able to dream of having 4 or 5 kids in a fairly sane 1950's-style society and economy.