alt Hacker NewsTL;DR:
Study claims iPhone contributed to a significant decrease in the birth rate after its release in 2007, when AT&T was the only carrier for the phone, allowing researchers to “isolate an iPhone-specific channel” and compare birth rates in areas with a high AT&T customer base to competitors' areas:
“The diffusion of the iPhone explains 33-52 percent of the decline in the general fertility rate among women aged 15-44.”
Authors go on to muse that “as modern smartphones diffused, time spent with friends in person and sexual activity fell sharply alongside rising consumption of pornography, a possible substitute for partnered sex.”
Nothing to do, of course, with AT&T’s customer base at the time being urban, well-educated, and white, or that U.S. birth rate in the youngest groups had already been falling before 2007 with the trend continuing during study period.
The authors do address this issue, by reweighting their treatment and control counties on observable covariates. But I agree with you that this isn’t the causally watertight research design that economists usually strive for.
It might be worthwhile using local lightning strikes as an instrument for 3G coverage. Others have done this, but not for fertility afaik. But the lightning strike data costs about $1000.