Stone credited the birth rate decline among all groups mostly to changes in marriage and marital status.
"Births to never-married women are down more than births to ever-married women," he said.
Since 2007, the age-adjusted fertility rate for married women is down 14 percent, while the never-married fertility rate is down 21 percent.
The statistics indicate the birth rate is falling more slowly for women with graduate degrees than women with bachelor's degrees, while the birth rate is falling most for women with no bachelor's degrees.
"Fertility declines are most strongly associated with factors that are race- or region-specific, not broadly class-specific, as different economic classes appear to have quite similar trends," Stone said. "This doesn't rule out all economic causes: there are important interactions between race and socioeconomic class."
He suggested that economically-oriented solutions may have only "modest direct effects" on the birth rate.
The CARA research blog, edited by Gray, took a look at a similar time period, 2010-2016. It found a net loss in the U.S. Catholic population of 0.9 percent.
"This is a dynamic that is happening at the level of the family where it meets the parish community. Something is disconnected," Gray said in a March 12, 2018 post.
Decline in marriage rates between Catholics and non-Catholics also mean a decline in non-Catholic spouses who convert to Catholicism. In 1996, 31 percent of all marriages were between Catholics and non-Catholics, compared to only 23 percent in 2015, Gray said.
"The most common reason given by adults converting to Catholicism for switching their religion is that they are marrying a Catholic. Fewer marriages in the Church between Catholics and non-Catholics will result in fewer adult entries into the faith."
The retention rate among Hispanic Catholics appears to be slipping.
(Story continues below)
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In 2010, 77 percent of Hispanics who were raised Catholic remained Catholic when surveyed, compared to 64 percent of non-Hispanic Catholics. By 2016, only 69 percent of Hispanic Catholics remained Catholic, compared to about 63 percent of non-Hispanic Catholics.
In 2010, 63 percent of all Hispanic adults in the U.S. self-identified as Catholic, compared to 54 percent six years later.
"Declining affiliation among Hispanic Catholics should be of great concern to the Church because a majority of Catholics under the age of 18, those of the iGen, are Hispanic," said Gray, referring to the generation after the Millennials as "iGen."
He suggested that descendants of immigrants from predominantly Catholic countries often show diminishing religious affiliation over time.
"Coming from a very Catholic country to one with abundant religious pluralism … is a dramatic cultural change," he said.
The numbers could also reflect differences among Hispanics by national origin.