Researchers have long tried to untangle the complicated mix of economics, culture, education and contraception (or lack thereof) that leads to teenage pregnancy. Despite a decline in births to American teenage mothers over the past two decades, the United States stands out among developed nations in that its teenagers are much more likely to give birth than their peers in Canada, Germany, Norway, Russia (a country that is still advancing on the spectrum of development) or Switzerland.  A new study by Melissa S. Kearney, an economist at the University of Maryland, and Phillip B. Levine, an economist at Wellesley College, builds on their previous research looking at the link between income inequality and rates of teenage childbirth.