A common error is to conclude the study shows that religiosity causes higher teen birth rates. But correlation does not imply causation. It could be that higher teen birth rates cause religiosity. Or perhaps a third, unidentified factor causes both.
But isn't the strength of association still impressive? It is. But what if, as I just suggested, there are other variables involved? Such confounding variables (or confounders, as they are commonly known) can wreak havoc on this sort of analysis. Indeed, the authors of the study did adjust for median household income and abortion rate (both at the state level). But it is possible that other confounders are lurking. And unfortunately, we tend to forget entirely about the possibility of confounders when we hear about study findings.
Another error is to conclude that the findings directly apply to individuals. Here I will quote the authors directly:
We would like to emphasize that we are not attempting to use associations between teen birth rate and religiosity, using data aggregated at the state level, to make inferences at the individual level. It would be a statistical and logical error to infer from our results, “Religious teens get pregnant more often.” Such an inference would be an example of the ecological fallacy ... The associations we report could still be obtained if, hypothetically, religiosity in communities had an effect of discouraging contraceptive use in the whole community, including the nonreligious teens there, and only the nonreligious teens became pregnant. Or, to create a different imaginary scenario, the results could be obtained if religious parents discouraged contraceptive use in their children, but only nonreligious offspring of such religious parents got pregnant. We create these scenarios simply to illustrate that our ecological correlations do not permit statements about individuals.To err is human ...
My goal here has not been to criticize the authors of this study, nor the media. Rather, what I find remarkable is how such a simple statement—"states whose residents have more conservative religious beliefs on average tend to have higher rates of teenagers giving birth"—can be so easily misinterpreted, and in so many different ways! Does anyone know of any research about our tendency to overinterpret scientific findings? Of course, we'd probably overinterpet it.