It's been happening for years: When the collection basket is passed at churches, it's the older generations who are more likely to drop a donation inside.

Are young people just not generous?

The Barna Group recently decided to explore that question among practicing Christian adults in the U.S. and concluded that young adults indeed are a generous group — they just have an expanded view of "generosity." They're more likely to volunteer generously, offer hospitality to those in need or offer emotional support to others.

They simply don't have the money, or the inclination, to donate with pledge cards or consistent giving plans that lock them in.

"We heard from pastors who felt that millennials weren't giving, but that didn't mesh what we knew of that generation," said Brooke Hempell, senior vice president of research at Barna, a national religion research group based in California. "I think maybe they are doing the same things, but in different ways."

Only 13% of millennials and 6% of the even younger Generation Z donated money to their church or a cause on a frequent basis, the study found. That compares with 26% of baby boomers and 41% of "elders" born before 1946.

The study found that when young adults do make financial contributions, it was often spontaneous and driven by compassion for the specific cause. But it was often financial sacrifice, something older donors reported, too.

"While pastors talk about giving generously to the church, they rarely talk about other aspects of personal finances, such as saving for the future, debt management and wise investing and planning," Hempell said.

"In fact, pastors are 3½ times more likely to talk about church giving than to address personal debt," she said, "which the data tell us is a burden in the lives of many Christians, particularly those in early stages of building career and family."

About a third of young adult Christians characterized their giving as "completely spontaneous," compared with just 7% of the so-called "elders" born before 1946. The elders' giving was nearly the opposite; more than a third said they donated money on a planned, consistent basis.

The data point to evolving approaches to generosity in society, offering information that will be useful to faith groups and nonprofits across the country, said Callie Briese, director of external affairs for the Minneapolis-based Thrivent financial services, which commissioned the study.

"We partnered with Barna to better understand how generations are talking about generosity," Briese said, "and to help churches and communities understand what motivates people, how people want to give."

She said the communities served by Thrivent, including religious and nonprofit organizations, are particularly "enriched by the spirit of generosity, whether it's people's finances, time or talents." So the study dovetails with Thrivent's mission.

"Our perspective is that when people are generous, they make financial decisions that align more closely with their values," Briese said.