Pollsters have been asking Americans questions about God, sex and babies for a long time and the answers used to be pretty predictable. 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 and "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."

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, and 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, 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 continued 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 Latinos.

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 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.

It's hard to pin political or cultural labels on some behaviors that are inspiring so many people to avoid marriage, to marry later, to have fewer children or to have their children later in life. At one end of the cultural spectrum is the 30something male whose solo life remains focused on his Xbox. At the other end is the professional woman working 70-hour weeks while striving to rise in a major law firm, even as her biological clock ticks loudly.

Of course, it also matters that children are expensive. In his book, Last examines a variety of expenses and career realities and concludes that it costs about $1.1 million to raise a single child, with home costs and college expenses higher in prime locations. When living in New York City, San Francisco or Washington, D.C., having two children is "having a lot of children," he said. "What's countercultural in one city is normal in another."

The bottom line is that Americans who choose to have large families are almost certainly making "some kind of theological statement," he said. "They are making countercultural decisions and people just don't keep taking specific countercultural actions without having some kind of purpose, a larger reason for what they are doing. ...

"Think of it this way. At some point, you have to ask: 'Am I the most important -- or even the only -- character that matters in the movie of my life? ... Parents just can't think that way and the more children you have the less you can afford to think of yourself as the center of everything that happens in the world. ... That's a very important lesson to learn about life."