Theories on the reasons for the dramatic shift include everything from new approaches to sex education to the widespread availability of broadband Internet. But most experts agree on the two major causes.

The first is the most important and may be obvious: Today’s teens enjoy better access to contraception and more convenient contraception than their predecessors, and more of them are taking advantage of innovations — such as long-acting injectable and implantable methods that can last years — over a daily birth control pill. But the second cause is something that goes against the conventional wisdom. It’s that teens — despite their portrayal in popular TV and movies as uninhibited and acting only on hormones — are having less sex.

“There has been a change in social norms that has happened in the past 20 years, and the idea of not having sex or delaying sex is now something that can be okay,” said Bill Albert, chief program officer for the National Campaign to Prevent Teen and Unplanned Pregnancy.

Veronica Gomez-Lobo, director of pediatric gynecology at Children’s National Medical Center, said the trend of abstinence has been mostly among younger teens rather than older ones. While there’s not good data on why this is happening, she thinks of it as a “contagion” factor: So many teens are waiting to have sex, she suggests, that the peer pressure goes opposite to the way that it might have in the past.

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“We think this is a very healthy trend,” Gomez-Lobo said.

The decline in birthrates has been going on for most of the past decade but appears to be accelerating. The issue has been important to President Obama, who in 2010 launched a $110 million initiative to scientifically validate prevention programs that work and to replicate them throughout the country.

The nation’s teen birthrate peaked in 1991, a time when posters of sad, pregnant girls were plastered on buses and subway stations and when popular culture was filled with references to “babies having babies.” The alarm was backed by evidence showing that having unplanned children at a young age carries numerous negative health and social consequences.

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Over the next 23 years, the birthrate plummeted 60 percent, from 61.8 births per 1,000 in 1991 to 24.2 births per 1,000 in 2014 — the lowest rate on record. Yet even with the dramatic improvement, studies still estimate that teen births cost taxpayers $9 billion each year.

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It’s impossible to talk about unwanted teen pregnancies without the subject of abortion coming up. While the new CDC report did not address this issue, research by the Guttmacher Institute shows that the decline in births is likely to be unrelated to more terminated pregnancies.

Researcher Isaac Maddow-Zimet said Thursday that teen pregnancies have been declining for 40 years, which by itself could explain the falling birthrates. Likewise, an analysis of 2011 abortion numbers, which represent the most recent and comprehensive data set available, shows that abortion rates in every state but Vermont decreased or remained the same from the previous year.

“Historically, abortion rates and birthrates have not always moved in parallel, but in recent years they have been doing so more,” Maddow-Zimet said.

Various surveys and studies have tried to quantify the impact of different interventions in helping reduce teen pregnancies and birthrates.

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One of the most interesting possibilities has been the popularity of MTV’s hit reality show “16 and Pregnant.” The struggles of the young moms in the show — who were often shown in tears —may have served as cautionary tales to millions of viewers their age. A study that came out in 2014 estimated that teen births dropped 6 percent in the 18 months following the show’s first broadcasts.

Another intriguing study looked at the impact of the Internet and concluded that at least 13 percent of the total decline between 1999 and 2007 might be explained by the increasing availability of broadband Internet. Researchers theorized that being online could help provide teens other means of exploring relationships and finding advice about effective forms of contraception, in addition to obtaining information about options for ending unwanted pregnancies.

Sex education programs have also changed a lot over the years. While many programs still use the traditional bananas and condoms that lead to so many giggles and red faces, new curricula also incorporate lessons on financial responsibility or focus more on overall development of a child’s character, approaches that some research has shown may be more effective.

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Albert and other experts think the economic downturn that started around 2006-2007 may have played a role, too. “They are not checking their stock portfolio before they hop in the sack, but teens are keen observers of the world around them, and if their own family had to do more with less, it may have fostered less risk-taking and a more cautious attitude about sex,” he said.

But while the overall national trend is positive, the CDC analysis of teens ages 15 to 19 shows that large racial and ethnic, regional and socioeconomic disparities remain. The birthrates for Hispanic and black teens, though lower than in the past, still are twice as high as that of white teens. In some states, the difference is even greater. In New Jersey, for example, the birthrate among white teens was 4.8 births per 1,000, which is well below the national rate of 18 for that group. But the birthrate among New Jersey’s black teens was 27.4 and among its Hispanic teens 31.3 — an almost seven-fold difference.

In addition, some counties still have pockets of high birthrates — even in states with overall low birthrates — and many of them are clustered in the South and Southwest. The CDC also noted that the places with the highest birthrates tend to have higher unemployment, lower income and less education. Teen birthrates in 2013-2014 at the county level across the nation ranged from 3.1 to 119 per 1,000.

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“The United States has made remarkable progress in reducing both teen pregnancy and racial and ethnic differences," CDC Director Tom Frieden said, "but the reality is, too many American teens are still having babies.”

This post has been updated.