The authors say:
International comparisons suggest that contraceptive use among U.S. teens is lower than in other developed countries (although the data available for such an exercise is somewhat limited), and this more than offsets the lower rate of sexual activity among U.S. teens, leading ultimately to a higher birth rate for U.S. teens.The study, however, focuses on cross-State variations in the US.
Motoko Rich from Economix explains the authors interpretations of the results:
Inequality, the authors suggest, makes the poorest citizens believe that they have little chance of economic mobility. They are giving birth “at a young age instead of investing in their own economic progress because they feel they have little chance of advancement,” the authors write. [Very controversial].