Since 1990, the fraction of Americans with no religious affiliation has nearly tripled, from about 8 percent to 22 percent. Over the next 20 years, this trend will accelerate: by 2020, there will be more of these “Nones” than Catholics, and by 2035, they will outnumber Protestants.
The following figure shows changes since 1972 and these predictions, based on data from the General Social Survey (GSS):
The GSS, which surveys 1,000–2,000 adults in the U.S. per year, includes questions related to religious beliefs and attitudes. Regarding religious affiliation, it asks “What is your religious preference: is it Protestant, Catholic, Jewish, some other religion, or no religion?”
In the figure, the dark lines show the fraction of respondents in each group for each year of the survey until 2016. The shaded areas show predictions, based on a statistical model of the relationship between year of birth, age, and religion.
Religious beliefs are primarily determined by the environment people grow up in, including their family life and wider social influences. Although some people change religious affiliation later in life, most do not, so changes in the population are largely due to generational replacement.
We can get a better view of generational changes if we group people by their year of birth, which captures information about the environment they grew up in, including the probability that they were raised in a religious tradition and their likely exposure to people of other religions. The following figure shows the share of people in each religious group, for birth years from 1880 to 1995:
