"When you take children out of the picture, the links binding people to churches or to institutions decay quite rapidly," he said.
As the connection to the faith declines, people also become more willing to vote in favor of issues opposed by the Church, such as contraception and abortion, he observed.
"So maybe fertility drives the faith decline. You could also argue that a decline in institutional religion makes people less prone to follow traditional ideas of what children are for, having lots of children to carry on the faith and so on."
Do pro-family policies work?
In Europe, some countries are trying to address the low fertility problem by introducing policies offering financial incentives for women to marry younger and families to have more children.
Hungary is one country leading the way in these kinds of policies, and it has had minimal success: its national statistics office estimates it has raised its number of births per woman from 1.23 in 2011 to 1.48.
Jenkins agreed that pro-birth policies can work at raising fertility rates, but he said they work very slowly and are very expensive. In the past, oppressive policies under dictatorships have shown the most impact, he explained. But in a democracy, the incentives to have children are financial and it is "phenomenally expensive to promote any significant change in the birth rate."
Italy has introduced some less aggressive policies, such as a "baby bonus" and subsidized parental leave, but one family policy expert said the truth is that they have not had much success in increasing births.
Vincenzo Bassi is a professor of law, economy, and political science in Rome. He is also the president of the Federation of Catholic Family Associations in Europe (FAFCE), an umbrella organization that gives support to Catholic families and promotes discussion of family policy issues within European institutions and local governments.
FAFCE tries to show policymakers "that the family is crucial for economic development," Bassi said. "Also demographic policies must be regarded as an investment because without children, without future workers, we cannot maintain the generational balance which is essential for the future, the economic future of Europe, of my country, and of the whole world."
Pro-family policies are only minimally effective, he said, because "if you don't have any vision, a vision pertaining to the role of the family in society, of course, these policies are just social policies, welfare state policies, emergency policies, but they don't have any real impact on the birth rate."
"If you don't realize the function and the role of the family in society, all of these policies are something OK, they can be useful," he continued, "but I don't decide to have more children because I'll have a [financial] bonus."
Having children requires a lot of sacrifices, Bassi noted. If we want to encourage people to take on that sacrifice, the family needs to be valued by society at large, he said: "I have to be happy, I have to feel important, having a family."
A very different world
In Bassi's opinion, where Italy should go from here is a complex question, but the family needs to have a greater role in both society and Catholic parishes and communities.
FAFCE promotes the formation of associations of families in parishes, as a means of providing mutual support and friendship.
"We need the generative power of the family not only within the family but also outside," he said. In a time when people no longer have the support of living close to extended family, "the first community is the parish."
"If we will start [making] this change also in the Church, we can hope that we can export the model outside the Church," he said.
As demographics continue to shift over the coming years, religious groups have to figure out "how to deal with a different demographic profile, of a society with a lot of lone adult singles of all ages," as well as a "very sharp increase" of old and super-old people, Jenkins said.
Religions have to recognize "the very different social and demographic world" they are operating in. "For many years, consciously or otherwise, churches, especially in the United States, have presumed that the normal population they are serving is based on families, nuclear families," but this just is not the case anymore, he said.
The Italian demographer Volpi was not optimistic about stopping or reversing the fertility trend, but he said that the Catholic Church should encourage reflection on how to exit the crises of marriage and the family.
"Because if you don't overcome the crisis of marriage, you don't overcome the crisis of the family, that is the discussion a bit," he said. "And you don't recover from the crisis of fertility either."