In his research, Stone highlighted the importance of distinguishing between religious membership- or even religious attendance- and religious belief. He warns that church attendance is not the best predictor of "religiosity."
Although over 80% of Americans will say they believe in God, only a third will actually attend church, he said.
Similarly, though not a large number of people regularly went to church before 1930, almost all would say they believed in God, Stone argued.
Stone also pointed out that church membership- the kind that is officially recorded- also is not always the best predictor of "religiosity," though it is helpful to observe as a "minimum level of behavior."
"A person baptized, married, and eulogized in a church is properly counted as part of a religious community, but nonetheless their experience of religion is different than someone who attends every week," Stone noted.
Stone pointed to several U.S. policy decisions that he believes have had an effect on the post-1960 decline in church attendance.
Among the policies he identified are Blaine Amendments, which grew out of 19th-century anti-Catholic sentiment and sought to prohibit direct government aid to religious schools. Today, 39 states formally restrict using any taxpayer money for religious instruction.
It was not until the mid-20th century that public education began to become as thoroughly secularized as it is today, Stone said. The rise of secular, public schools and the decline of religious schools in the U.S. meant that students who attended public schools after the 1940s "spent much of their life in schools that were far more secularized, and these are the generations during which religiosity has declined."
Changing family dynamics, including an increase in the average age of marriage, also have had an effect on religiosity, Stone said.
He contended that a greater emphasis on higher education- which takes years to complete- has led to more people delaying marriage or choosing not to get married at all, meaning they are less likely to form religious habits such as attending church.
Additionally, a rise in interfaith marriages plays a role, Stone said. The children of interfaith marriages are less likely to adhere to either of their parents' religions, or any religion, than children whose parents share the same religion.
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