America's teen birthrate has hit a new low, marking the seventh straight year of decline.

New federal data shows that in 2015, there were 22.3 births for every 1,000 girls between 15 and 19. That's an 8 percent decline from 2014 — and a 43 percent decline since just 2007.

That's remarkable: In less than a decade, the United States' teen birthrate has fallen nearly in half.

Teen births peaked in 1991. Ever since, they've been on the decline.

In the late 1980s and early '90s, teen pregnancy seemed like a national crisis. In 1991, the year the teen birthrate peaked, there were 61.8 births for every 1,000 teenage girls. Time magazine devoted its cover to the issue in 1985:

The topic factored heavily into electoral politics at the time. Bill Clinton's reelection campaign had an entire white paper devoted to the issue of reducing teen pregnancy.

"To strengthen the family we must do everything we can to keep the teen pregnancy rate going down," Clinton said in his 1996 State of the Union Address. "I am gratified, as I'm sure all Americans are, that it has dropped for two years in a row. But we all know it is still far too high."

What Clinton didn't know at the time was that the country's teen birthrate was at the start of a multi-decade decline that would stretch up through 2015.

Since the mid-2000s, the decline has become especially steep, falling by as much as 8 or 9 percent per year.

This drop occurred in tandem with steep declines in the abortion rate. That suggests that the drop isn't the product of more teenagers terminating pregnancies. It's simply that fewer girls are getting pregnant.

The many theories of the teen birthrate decline

Those who study teen pregnancy know that teen births are declining quickly — but they still don't understand exactly why this is happening. They do have a lot of theories, something I've covered previously for Vox.

Some of those theories don't hold up so well a year or two later. For example, many used to argue that the depressed teen birthrate might reflect the recession leading to more cautious decisions about childbearing. But here we are in 2016, with the economy recovering and the teen birthrate declining as quickly as ever before.

But other theories still stand up quite well. Here is a summary of the leading contenders:

Teenagers are increasingly using the best types of birth control. Separate federal data shows that more and more teens are using long-acting reversible contraceptives like implants and intrauterine devices (IUDs). These types of birth control have very low failure rates.

Separate federal data shows that more and more teens are using long-acting reversible contraceptives like implants and intrauterine devices (IUDs). These types of birth control have very low failure rates. Teenagers today are much less exposed to lead than they were in prior decades. For decades now, researchers have documented a dangerous relationship between a child's early exposure to lead and his or her cognitive development. Research led by economist Rick Nevin argues that the decline in teen births dovetails quite closely with declines in children's blood lead level rates.

For decades now, researchers have documented a dangerous relationship between a child's early exposure to lead and his or her cognitive development. Research led by economist Rick Nevin argues that the decline in teen births dovetails quite closely with declines in children's blood lead level rates. Teenagers are ... watching a lot of Teen Mom? Yes, the MTV show. One 2011 paper estimated it and 16 and Pregnant (which show a realistic depiction of the difficulties of teen parenting) are responsible for a 5.7 percentage point drop in the teen birthrate between June 2009 and December 2010 — one-third of the total decline during that 18-month time period.

You can read more about these theories and others in the feature I wrote last January.

Fewer teen births is a huge public health win

The massive decline in teen birth rates is undeniably good news for public health advocates.

Teen mothers are significantly more likely to drop out of high school. One 2010 study found that more than 90 percent of women who did not give birth as teens obtained a high school diploma or GED by time they turned 22. For teen moms, that figure stood at 51 percent. Thirty percent of teen girls who drop out of high school cite pregnancy as the reason.

Most teen mothers do not receive financial support from their child's father; 48 percent live below the poverty line. Avoiding early motherhood undeniably opens additional doors in a teen's future — and in her child's.

Individuals born to teen mothers tend to have lower earnings and report less satisfaction with their lives than those born to an older parent. They are also more likely to become teen parents themselves.

And that's why the teen birthrate decline is such good news: It opens economic doors for teens that parenthood can often close, both for themselves and their eventual children.