When is it OK to juxtapose historical trends?
You have to watch out for this (via Boing Boing):
There is no reason to suspect that the rise in autism is linked to the rise in organic food sales.
But other times it seems reasonable to me, like this:
There are lots of reasons the long-run decline in fertility is related to the rise in women’s employment rates. We know from lots of research that women with more children are less likely to have jobs; women with jobs are less likely to have children; and over time the proportion of women in the second group has grown relative to the first.
So it’s OK to use eyeballed historical trends when you have good research backing up the association. Your conclusions, then, don’t rely on the simple trend comparison. The trends are an illustration.
But trends need not have have a simple cause-and-effect relationship — or a unidirectional relationship — for it to be important to compare them. Sometimes the relationship is just descriptively important. So, looking at the graph above, it would be reasonable to say, “Women’s lives sure have changed. They have fewer children and more jobs, on average, than they used to.”
And then there is the negative case. The other day I complained when Kay Hymowitz implied that the rise in father-absent families caused an increase in crime among boys. And I offered this simple trend comparison to undermine that story:
It was not my intention to say there is no connection between father absence and boys’ criminal behavior. (I’ve sketched out some possible links in this old post; and made essentially the same comparison about single mothers and crime before.) But the lack of a strong correlation in the trends over time is a challenge. That’s what I’ve been arguing about cell phones and traffic accidents:
Of course driving and texting is dangerous. And of course single parents have a harder time (on average) supervising and disciplining their children than married parents. But if there is a big discordance between the trends — texting and driving, single parents and crime — then that’s a problem for the story that one trend is driven by the other. Causal relationships may be apparent in a lab, or at the margins, but to explain large-scale social change is more difficult — and that’s often what we’re trying to do when we draw from specific research to make political, policy, or theoretical arguments.
So, it’s OK to use discordant trends to take potshots at a proposed causal story, to express skepticism. The discordant trends are a hurdle for the theory to overcome.
If you have good research showing that single parenthood, and especially father absence, is harming boys more than girls, then it would be to OK to use trends as an illustration. It just can’t be your main evidence. So Kay Hymowitz could reasonably include a graph like this to accompany her extensive review of the research on family structure and trouble for boys:
Yes, women’s advantage in high school and college completion has accompanied the trend toward father-absent living arrangements for young boys. That doesn’t fulfill her need to present more direct evidence, but it’s a piece of supporting evidence.
Conclusion: Juxtaposing historical trends is not how you prove a theory. It is a great tool for illustrating known associations, for describing social change, and for challenging theories or narratives.