The sharp decline of teen pregnancy in the U.S. isn’t just one of the great (and underappreciated) public health victories of the past few decades—it’s also a budget victory.

As the Centers for Disease Control and Prevention explains in a report this week, the teen birth rate has nosedived 57 percent since 1991. The total number of children born to adolescent mothers is lower today than it was in 1950, when the country was a bit less than half the size it is today.

Teen mothers are especially likely to use safety-net programs like Medicaid and WIC (which provides food for new moms and their infants), so as their numbers have shrunk, taxpayers have saved money. The CDC cites survey research by the National Campaign to Prevent Teen and Unplanned Pregnancy, which estimated that federal, state, and local governments avoided spending $12 billion in 2010, thanks to the post-1991 drop.

But why are teens having fewer children? That’s a complex social and economic question that academics are still trying to break down. (Sometimes the research leads to surprising results: One study suggested MTV shows such as 16 and Pregnant and Teen Mom may have played a role in declining rates.) Since the early 1990s, when births began their drop after rising for several years, the fraction of teens having sex has fallen while contraception use has become more common. That’s given ammo to fans of abstinence-only education and safe-sex-education advocates alike. It’s also possible that the recession helped hasten the fall of teen births after 2007. (Fertility fell pretty much across the board after the meltdown, just like employment.)

For its part, the CDC cites one telling paper from the American Journal of Public Health. Using government survey data on adolescent sexual behavior, it concluded that 86 percent of the decline in teen pregnancy between 1995 and 2002 could be chalked up to increased contraception use; the other 14 percent was due to abstinence. “The decline in U.S. adolescent pregnancy rates appears to be following the patterns observed in other developed countries, where improved contraceptive use has been the primary determinant of declining rates,” the researchers wrote.