“The share of the U.S.’s 130 million households headed by married parents with children under age 18 fell to 17.8% in 2021 from 18.6% last year,” according to Bloomberg’s summary of census data. “That’s down from more than 40% in 1970. . . . By absolute numbers, there are just 23.1 million homes with nuclear families, the fewest since 1959.”