Vox's Sarah Kliff talked to social scientists about changes in the U.S. that could have led to these declines -- especially the 36 percent drop in teen births since 2007. She found several interesting hypotheses (increase in birth control access, better sex education, etc.), but none completely explains the movement. It can be frustrating to hear questions without answers, but the Vox story is a satisfying read. It's a traipse through some of the major recent forces in American culture, with a stop at 16 and Pregnant along the way.