I fear that the new cellphones-cause-low-fertility NBER working paper has found a potentially genuine divergence between the fertility rate in rural and urban counties in the US, then labelled it "iPhone."
The problem is that AT&T coverage was heavily urban, resulting in an imbalance between the control and treated group.
The matched Poisson regression is supposed to address that by overweighting the small number of notionally urban counties that did not have iPhone coverage. But I believe that by "urban," they mean counties in which just 33% or more of the population were in towns of 2,500+ people. It would therefore included a lot of places that are, in fact, rural.
To me, it seems like there's a failure of identification.
Hence, the SDiD result could reflect iPhones or a differential effect of the Panic of 2007 on rural and urban areas. We don't know.
The other-carrier placebos are also uninformative because they are structurally incapable of reproducing the urban-rural confound, given how they are mapped.
The paper is here:
nber.org/papers/w35310.
