It was one of those happy social-media pictures, only this time the pregnant mother was celebrating with her nine children.

Los Angeles comedian and actor Kai Choyce was not amused and tweeted the photo with this comment: "this is environmental terrorism. … In the year 2020 literally no one should have ten kids."

The result was a long chain of sweet or snarky comments, as well as photos of large families. One tweet quoted a Swedish study claiming that having "one fewer child per family" can save an average of 58.6 tons of "CO2-equivalent emissions per year."

Debates about fertility often veer into fights about religion and other ultimate questions, such as the fate of the planet. 

Parents with two-plus children are often making a statement about the role of religious faith in their lives. People on the other side of this debate have frequently rejected traditional forms of religion. 

"What we call 'culture wars' are wars about demographics, but we have trouble discussing that," said historian Philip Jenkins, who is best known for decades of research into global religious trends, while teaching at Pennsylvania State and Baylor University. His latest book is "Fertility and Faith: The Demographic Revolution and the Transformation of World Religions."

In the 1970s, researchers thought the link between secularization and falling birth rates was a "Protestant thing" in Europe, but then this trend spread into Catholic cultures in Europe and in Latin America, he said. Fertility rates are now collapsing in Iran and some Islamic cultures. Meanwhile, Orthodox Jews and traditional Catholics continue to have larger families than liberal believers in those ancient faiths.

America's 2019 birth rate fell to 1.71, its lowest level in three decades, and well under the replacement rate of 2.1. This took place before the coronavirus pandemic and the Brookings Institute recently predicted a "COVID baby bust" next year, resulting in up to half a million fewer births.

Researchers frequently argue about which comes first -- secularization or declining fertility.

"I'm not sure that really matters because these two trends are so clearly related that they just march along together," said Jenkins. "If you tell me the fertility rate in any given culture, I'm going to be able to predict the status of religious life there -- in terms of people supporting religious institutions and practicing a religious faith."

In recent decades, researchers have noted that the more citizens practice traditional forms of faith -- worship attendance levels are crucial -- the more likely they are to support politicians who are conservative on moral and cultural issues. This is frequently called the "pew gap." 

This trend has helped fuel conflicts between urban Americans along the East and West coasts and those living in smaller cities and rural areas in the heartland.

During national elections, Jenkins added, "studying the fertility rates in specific states usually tells you what will happen" in voting booths, he said. Clashes over religious and cultural issues "almost always translate into political behavior."

When studying ties between "secularization" and fertility rates, Jenkins stressed that it's important to focus on whether people actively support religious institutions and strive to practice specific doctrines. More and more Americans will "say that they have religious beliefs, of some kind of another, but they no longer choose to belong to traditional religious groups" or follow specific doctrines.

Thus, it's crucial to note the continuing surge in the number of religiously unaffiliated Americans who identify as "nothing in particular" or "spiritual, but not religious." Last year, the Pew Research Center updated its research on this subject -- noting that 26% of Americans are so-called "nones," up from 17% one decade ago.

The bottom line: Changes in religious practices and beliefs are -- in America, and around the world -- having practical effects in bedrooms, nurseries, sanctuaries, voting booths, classrooms and elsewhere. 

"When you disconnect marriage, sexuality and children you also start redefining the meaning of family and all that goes with that," said Jenkins. "That's a perfect example of how changes affecting religious institutions and doctrines turn into conflicts that affect politics and culture. …  

"You don't see the kinds of culture wars that we see all around us without these kinds of demographic trends running in the background, changes based on how people are actually living their lives."