Most of the drop occurred in the late 50s to mid 60s. Fertility in the US peaked at over 3.5 births per woman in the 50s before falling below 2 by the late 60s. It then stabilized for 40 years and even recovered to the low to mid 2s in the 90s and 2000s before falling post-2008 from 2.2 to 1.7.

Cell-phones. Noone’s having sex because they’d rather watch Tik-Tok; it’s got lower friction than going outside.

perhaps religious and moral pressure not to excersize the choice available has decreased between 50s and 2000s