TL;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.