Fewer adults in developed countries are tying the knot or having kids -- a trend that’s causing furrowed brows in religious circles and pro-marriage crowds. Pope Francis last month lamented today’s “culture of temporary,” warning guests at a Vatican conference that eroding traditions could lead to “spiritual devastation.” But declining marriage and birth rates could have broad economic impacts as well, as adults delay buying homes, saving for retirement and investing in the permanent.

“We’re a less marriagecentric society than we used to be,” said Isabel V. Sawhill, an economist and co-director of the Center on Children and Families at the Brookings Institution in Washington. As a result, “there’s going to be less investment in the long term, like in housing, furniture and other consumer durables,” she said. By forgoing mortgages and college funds for kids, unhitched and childless adults will instead spend more on short-term fixtures, such as vacations, restaurants and apartment rentals, she said.

The U.S. marriage rate in particular has dropped to its lowest level in nearly a century. Just 50.3 percent of the adult population was married in 2011, compared with the peak of 72.2 percent in 1960, according to a Pew Research Center analysis.

Across Europe, the rate of knot-tying is about 40 percent lower than it was in 1970. The region saw 4.8 marriages per 1,000 people in 2008, down from 7.9 marriages four decades earlier, according to the latest available data from Eurostat, a European Union statistics agency. Australia, Japan, South Korea, Brazil, Mexico and other industrialized economies have similarly seen their marriage rates decline.

The falling numbers are largely rooted in decades-long societal changes. For example, as more women enter the workforce, they’re less dependent on finding a partner for economic stability. More couples are comfortable with ditching the courthouse and “cohabitating” as unmarried partners, even in strongly Catholic countries such as Spain and Italy.

But the latest drop in global marriages is also partly due to the economic recession. As unemployment rises across the world, and college debt balloons in the U.S., the idea of settling down and starting a family is comparatively less appealing for many people in their 20s and 30s now than in the past. In European countries, the recession is causing more young people to stay at home, and nearly one-half of people between 18 and 30 still live with their parents, according to EU data cited by the Guardian.

“This youngest generation is feeling very uncertain about the economy, about the stability of their relationships and the stability of the jobs that they hold,” Sawhill said. “And this is going to make some much more risk-averse.”

That economic uncertainty is pushing down birth rates in the developed world, although numbers have also been declining because more people are choosing careers and leisure over children or opting for smaller broods.

For instance, Alexia, a 28-year-old Greek photographer who lives in London, said she would eventually like to have kids. “But, at the moment, I am not financially where I’d like to be in order to raise a family, and I feel I don’t want to be tied down,” she said. “I want to travel and do some projects before I have a baby.”

Over the past four decades, total fertility rates in the 34-nation Organization for Economic Cooperation and Development, including the U.S., have dropped by nearly 40 percent. In 2010, 1.7 children were born for every woman of childbearing age, compared with 2.7 children in 1970, OECD data showed.

Edward Yardeni, an economist in New York, has argued that a rise in single, childless adults could be good for some aspects of the economy. “Selfies,” as he calls the group, are more flexible and thus can seek out jobs in multiple markets, which allows companies and industries to grow quickly and efficiently, Yardeni said in a recent client note cited by Bloomberg News.

But there is an economic downside for singles, he said. Folks who rely on one source of income or health insurance are more vulnerable to layoffs and illnesses. Fewer kids in the world also means fewer future taxpayers, resulting in a smaller social safety net to support people as they age, he said.

In the U.S., in particular, legal and financial systems tend to favor married couples. A 2013 analysis by the Atlantic magazine found that more than 1,000 U.S. laws offer overt benefits to married partners, while a bevy of insurance policies, including for health, life and home, cost less for married people on average than for unmarried individuals.

For María Teresa Castro-Martín, however, the decision to delay having kids is a larger economic indicator than the decision not to marry. Castro-Martín is a research professor in the population department of the Spanish National Research Council, a government institute in Madrid. “I think it’s less a question of if you’re married or not, and more a question if you have kids or not,” she said by phone.

She noted that in Spain and other developed countries, more couples are opting to have children out of wedlock, a decision that drives them to make long-term financial choices, such as buying houses, purchasing better cars and investing in savings plans -- even in the absence of marriage certificates.

Alexia, the Greek photographer, agreed that wedding vows aren’t a critical factor for starting a family. “I think having a family can equally tie a couple together without the marriage,” she said. “I just feel that marriage might not be necessary.”

Some 36 percent of Spanish children were born to unmarried women in 2012, up from just 4 percent in 1980, Castro-Martín said. “These aren’t adolescent mothers, or single women. They’re just children of unmarried couples. Things have simply changed,” even in a traditional society like Spain, she said.

In all of Europe, the percentage of nonmarital births is about 39 percent on average, while the figure in the U.S. is 41 percent, according to Pew data.

Castro-Martín said such shifting attitudes among young adults are likely here to stay even as the economy improves. While more couples might decide to tie the knot or start families in a brighter job market, marriage and birth rates will likely never return to 1970s-era levels.

“I don’t think the trend will reverse; it’s a general trend across all of the world,” she said. “We won’t return to the numbers of the past.”