Teen fertility collapsed globally starting around 2007. This affected countries across the income and policy spectrum. This paper argues that smartphones changed how teens spend time with each other, and that this change in turn drove the collapse in teen fertility. Once enough teens are on the phone, being on the phone is where the peer network is; in-person time falls sharply, and with it the unstructured contact in which most unintended teen conceptions occur. A coordination model formalizes ...