Umbrellas, soaked heads, and the ecological fallacy

This is the second of four posts on religiosity and life history. In yesterday’s episode, I noted that the evidence in favor of viewing religiosity as driven by fast life history comes from group-level data. Poorer nations tend to have more churchgoers; American states and racial groups with more teen births and lower socioeconomic status contain more churchgoers.

A key risk here is the ecological fallacy—that is, the mistake of drawing individual-level conclusions from group-level data. It seems natural to conclude, for example, that if states with higher teen birth rates also have higher average church attendance rates, then surely churchgoers are more likely to be teen parents. But that’s the fallacy. You really don’t know what’s happening at the individual level until you check individual-level data.

Here’s a hypothetical. Imagine that you were curious about the effectiveness of carrying umbrellas (vs. not carrying umbrellas) in preventing people’s heads getting soaked by rain. Seems like it’s pretty obvious, but you want to see some numbers. You don’t find any individual-level data on it, but you manage to find averages in various cities for (1) the percentage of people who habitually carry umbrellas and (2) the percentage of people who often get their heads soaked by rain. So you decide to correlate those two city-level variables.

Here’s the thing: This would probably produce a positive correlation between umbrella-carrying and head-soaking! There would be cities like Seattle and Glasgow where lots of people carry umbrellas and also lots of people frequently get caught unprepared in the rain. And there would be cities like Los Angeles and Barcelona where hardly anyone carries umbrellas and also people hardly ever get their heads soaked by rain.

The ecological fallacy would be to use this group-level correlation to conclude that carrying umbrellas makes individuals more likely to get head-soaked—if they just stopped carrying umbrellas, their heads would be dry.

Of course that’s silly. The real answer is that there are places where it hardly ever rains and thus there’s little need to carry an umbrella to keep your head dry. But in other places, it rains a lot. There, it’s the umbrella-carriers who have drier heads than non-carriers. The right data here is individual-level data that measures average local rainfall along with umbrella-carrying and getting head-soaked. That data will show what the city-level data can’t.

So, we know that places with increased levels of fast life history are also places that contain more churchgoers. But that’s simply not enough information to know whether individuals with fast life history are the same individuals attending services. Outcomes like teen births and low education could be the head-soaking or the rain, while church attendance could be the umbrella.

This scenario is related to what Baumard and Chevallier (2015) were suggesting. In their view, it’s the intermingling of fast and slow life histories that causes the slow folks to seek out religion. And as we head into highly developed societies—to return to the analogy—it hardly ever rains, so you just don’t find many individuals carrying umbrellas.

We’re getting closer now. What we need is to look at both the individual-level and group-level predictors of church attendance. Baumard and Chevallier think it’s going to be slow individuals in groups that also have lots of fast individuals. But now we run into another problem, one that infects many current discussions of life history in evolutionary psychology. The problem involves the relationship between fast/slow life history and unrestricted/restricted sociosexuality. This will be the subject of tomorrow’s post.

[Update: The next post in this series is here.]