S
ince 1990, the fraction of Americans
with no religious affiliation has near-
ly 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 first figure on the right shows
changes since 1972 and these predictions,
based on data from the General Social Sur-
vey (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 be-
tween year of birth, age, and religion.
Religious beliefs are primarily deter-
mined by the environment people grow up
in, including their family life and wider so-
cial 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, includ-
ing the probability that they were raised in
a religious tradition and their likely expo-
sure to people of other religions. The sec-
ond figure on the right shows the share of
people in each religious group, for birth
years from 1880 to 1995.
Among people born before 1940, a large
majority are Protestant, only 20–25 per-
cent are Catholic, and very few are Nones
or Others. But these numbers have changed
rapidly in the last few generations: among
people born since 1980, there are more
Nones than Catholics, and among the
youngest adults, there may already be more
Nones than Protestants.
However, this view of the data does not
show the effect of age. If religious affiliation
increases or decreases, on average, as people
get older, this figure could be misleading.
Fortunately, with observations over more
than 40 years, the design of the GSS makes
it possible to build a statistical model that
estimates the effects of birth year and age
separately. Then we can use the model to
generate predictions, by simulating the re-
sults of future surveys. The details of this
methodology are in a longer version of this
article (see links below).
Are These Predictions Credible?
Social changes are generally unpredict-
Religious affiliation, by year of birth
1900 1920 1940 1960 1980
80
60
40
20
0
Year of birth
Percent
Protestant
Catholic
None
Other
Protestant
Catholic
None
Other
Religious affiliation, by year of survey
80
60
40
20
0
Percent
1980 1990 2000 2010 2020 2030
Year of survey
CHARTS BY
ALLEN DOWNEY
