Pollsters have been asking Americans questions about God, sex and babies for a long time and the answers used to be pretty predicable.
Early in the 20th century, it was easy to predict which flocks of believers would produce the most children -- with Mormons reporting the highest numbers, followed by Catholics, then Protestants and so forth as fertility rates declined. But things changed as the century rolled on and America became more pluralistic and, in elite zip codes, secular.
After Woodstock and the Sexual Revolution, it was clear "what really mattered wasn't what religion you claimed to be practicing, but the degree to which you actually practiced it -- especially whether or not you were in a pew week after week," said journalist Jonathan A. Last, author of "What to Expect When No One's Expecting."
These days, people who attend worship services once a week or more have a sharply different fertility rate from those who avoid religious sanctuaries. "It really doesn't matter what kind of services we're talking about -- Catholic, evangelical, Jewish, Mormon, whatever. What matters is whether you show up," said Last.
The bottom line: An activity that encourages people to get married sooner, stay married longer and have a higher rate of happiness while married will almost certainly produce more babies.
"When it comes to people having what people today consider large families -- three or more children -- there are two Americas out there," he said. The division is between those who actively practice a faith, especially a traditional form of faith, and those who do not.
This is crucial information in an era in which declining birth rates affect debates about a wide array of hot-button cultural issues: from Social Security to national health care, and from immigration reform to the future of major religious groups.
The Centers for Disease Control and Prevention recently reported that U.S. births appear to be leveling off, although the numbers continue to show some decline. While birth rates edged up for women in their early 40s and throughout their 30s, rates kept falling for women in their 20s and among Latinas.
A key factor, Last explained, is "aspirational fertility," or the number of children that parents say they want to have. In the early 20th century, a clear majority of Americans favored having three or more children. Now, 66 percent of those who seldom or never attend worship services say zero, one or two kids is ideal, while 41 percent of those worshipping weekly desire three or more children. If a woman frequently attends worship services, it is much more likely she will have a larger family, if that is her goal.