America's Abortion Drop

Why fewer women are ending pregnancies

It's not every day that both sides of the abortion debate celebrate the same milestone. But that's what happened on Feb. 3, when a report was released showing that the U.S. abortion rate in 2011 had fallen to its lowest level since the Supreme Court legalized the procedure nationwide in 1973. Abortion-rights groups heralded the news as proof that wider access to contraception reduces the need for abortion. Anti-abortion activists, meanwhile, said it showed that more women are carrying unintended pregnancies to term on moral grounds.

But those are just two of many possible explanations. The authors of the study, funded...