By Bradley Wright
In sociological research, we often study the change of things over time. It’s reasonably easy to demonstrate how something changes over time, but it’s more difficult to explain why it changes because there are multiple causal processes that produce such change. In particular, three that are frequently described in demographic research are age, cohort, and period effects. I’ll illustrate each one using data about religious change over time.
An age effect is how people change as they get older. As people progress from childhood to adolescence to adulthood they go through various changes. Not only do their bodies change (as we start to lament once we hit middle age), but they change socially as well. For example, political beliefs can change with age; many people report being more liberal in their youth and more conservative in their older age. People tend to earn more money as they age too (at least until they hit retirement).
In terms of religion, it’s commonly observed the people become more religious as they age. To illustrate this, I examined data from the General Social Survey (GSS), a survey collected every two or three years from 1972 to the present. One of the questions asked if respondents identified with a formal religion, and, if so, which one. In the following figure, I chart how many of the respondents professed themselves to be Christians as a function of age. About 75% of the respondents in their twenties professed Christianity (the bar on the left), and this increased steadily until almost 90% of those in their seventies and eighties professed Christianity. This illustrates an age effect—adherence to Christianity increases with age.
Not only do people change with age, but they also change with when they were born. Different cohorts (or generations) act and believe differently than other generations.
Here’s an easy example: If you’re a college student, go find your parents’ high school yearbooks and get a laugh out of how they dressed. In the 1970s and 80s, boys wore their hair long, and girls had frizzy hair, and we all wore embarrassingly short shorts. If you want to see something really crazy, go back another generation and look at young people from the 1960s.
With religion, we might expect a similar effect. Maybe different generations experience religion differently, beyond how they age. To examine this, I used GSS data to look at the relationship between age and Christianity for three different birth cohorts—respondents born in the 1940s and the 1950s. The first graph replicates the graph above, levels of Christianity over age, for people born in the 1940s. The next for those in the 1950s. As you can see, the relationship between age and Christianity changes by cohort. The respondents who were born in the 1940s held steady in their profession of Christianity as they aged, but the later cohort actually declined somewhat.
In addition to age and cohort effects, there are also have history effects. Here society changes in some way that affects everyone. The classic example is the Great Depression of the 1930s. It affected people regardless of their age or their birth cohort. Various aspects of religion change with historical changes. Wars and other times of trouble might reinforce the strength of religious beliefs. The social turmoil of the 1960s led young people to question existing social institutions, including organized religion.
As a simple illustration of a historical effect, I took levels of professed Christianity by decade, for the 1970s, 80s, 90s, and the 2000s. As shown in the graph, a smaller percentage of the population is professing Christianity with each decade. (To be clear, since the population is growing, there are actually more
professed Christians in the country now than in the 1970s, they just constitute a smaller percentage.) This appears to be a historical effect—the times they are a changing.
If this weren’t confusing enough, age effects can change by both cohorts and periods. (In statistical terms, this is a called an interaction effect.) So, the relationship between age and Christianity now might be different than it was in the past. Who knows, maybe the relationship between age and cohort various by historical period.
The upshot of all of this is that it’s rather difficult to make sense of social changes over time, and this leads to a fair amount of confusion about what’s really going on in society. What seems to be an easy question, for example, changes in professed Christianity over time, is somewhat difficult to make sense of. In this post, I’ve only looked at one aspect of religion, but we could do similar analyses for other religions as well as other aspects of religion, such as belief in God, importance of religion, and attendance at religious services.
I suppose that this post serves as a warning about over-simplifying social trends. If nothing else, this difficulty will always give us sociologists something to do with our time.