"American fertility is roughly at the replacement rate, 2.1 children per woman. Nor does the U.S. rate merely reflect, as some think, a higher rate among Hispanic Americans. The fertility rate is 1.9 for non-Hispanic whites and about 2 for African Americans, reports demographer Nicholas Eberstadt of the American Enterprise Institute. What explains the American exception? Eberstadt cites three differences with Europe and most other advanced countries: greater optimism, greater patriotism and stronger religious values. There's some supporting evidence. A survey by the National Opinion Research Center at the University of Chicago asked respondents in 33 countries to react to this statement: "I would rather be a citizen of [my country] than of any other." Among Americans, 75 percent "strongly" agreed; among Germans, French and Spanish, comparable responses were 21 percent, 34 percent and 21 percent, respectively."
I think it is more about religious values myself (regarding which the US is a unique outlier for an advanced nation economically). The rest (including the patriotism thing) I suspect is fluff. So say this near atheist.
I should have pinged you to to my latest brain fart.