Summary so farIn Part One I described trends in market share of major religions in the U.S.: since 1988, the fraction of Protestants dropped from 60% to 51%, and the fraction of people with no religious affiliation increased from 8% to 18%.
In Part Two I used data from the 1988 General Social Survey (GSS) to model transmission of religion from parent to child, and found that the model failed to predict the decrease in Protestants and the increase in Nones that occurred between 1988 and 2010.
In Part Three I looked at changes, between 1988 and 2008, in the spouse tables (which describe the tendencies of people to marry within their religions), the environment table (which describes parents' decisions about their children's religious upbringing), and the transmission table (which describes the likely outcomes for children raised within each religion). I found that the transmission table has changed substantially since 1988, and accounts for a large part of the observed increase in Nones, but not the decrease in Protestants.
Part Four revisitedIn Part Four I looked at changes in religiosity over the lifetime of respondents. The GSS is not a longitudinal survey, so we can't follow individuals, but we can follow generations (which I partition by decade of birth) over time.
Last time I presented this figure, which shows religiosity (the fraction of respondents with any religious preference) as a function of respondent's age, partitioned by decade of birth, for people who were raised Protestant:
Each line represents a different generation. For example, the red line shows that people born in the 1920s were about 96% likely to report a religious preference when they were interviewed in their 40s, 50s, and 60s, and possibly less likely to be religious when they were in their 80s.
The conclusion I drew from this figure is that the differences between generations are larger than the changes, over time, within each generation. For purposes of modeling I concluded that religious disaffiliation accounts for only a small part of the observed changes in religious identity.
But I was bothered by one feature of these curves: many of them are concave down, and the maximum point in the curves is apparently shifting toward younger ages. I came to suspect that this picture of the data is "out of focus".
We can refocus the image by plotting the date of the survey (rather than the respondent's age) on the x-axis. Here's what that looks like:
In this figure, two trends are more apparent: before 1990, most generations were becoming more religious; after 1990, they all became less religious. So it seems clear that the explanation is something that affected all generations at a particular interval in time, not something that affects all people as they age.
We can see these changes more clearly by normalizing each curve with its 1990 value:
Here's the same graph for people raised Catholic:
The general shape is the same: religious affiliation was flat or increasing prior to 1990, and decreasing for almost all generations after 1990.
Since the trends are similar for Catholics and Protestants, we can get a less noisy picture by combining them. Here is the same graph for respondents raised with any religion.
This figures makes it easier to compare across generations. It appears that more recent generations (born in the 1960s and 1970s) are disaffiliating at higher rates than earlier generations.
[As an aside, this result contradicts one of the primary (and widely-reported) claims of this article: Schwadel, Period and Cohort Effects on Religious Nonaffiliation and Religious Disaffiliation. Schwadel reports that people born in the 1960s and 1970s were disaffiliating at a slower rate than the previous generations. Some reasons my results might be different: Schwadel only had GSS data up to 2006, and he discards people under 30 years of age. So very little data about the youngest generations is included. Also, his results are based on statistical models that (if I understand correctly) don't include time as an explanatory variable, so they cannot account for an event that affects all generations during a particular interval.]
All right, it's audience participation time. What happened in the 1990s that caused widespread religious disaffiliation? Remember, idle speculations only. No evidence, please!