The birth rate among American teenagers, at crisis levels in the 1990s, has fallen to an all-time low, according to an analysis released Thursday by the Centers for Disease Control and Prevention.

The decline of the past decade has occurred in all regions in the country and among all races. But the most radical changes have been among Hispanic and black teens, whose birth rates have dropped nearly 50 percent since 2006.

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 cause 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 like long-acting injectable and implantable methods 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, hormone-filled crazies – 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.

Albert called the milestone “one of the greatest success stories of the past two decades” in public health.

But while the overall national trend is positive, large racial and ethnic, regional and socioeconomic disparities remain. The birth rates for Hispanic and black teens, while lower than in the past, still are twice as high as that of white teens. In some states, the difference is three times higher.

In addition, some counties still have pockets of high birth rates – even in states with overall low birth rates – and many of them are clustered in the south and southwest.

The CDC analysis of teens ages 15 to 19 also noted that the places with the highest birth rates tend to have higher unemployment, lower income and less education.

“The United States has made remarkable progress in reducing both teen pregnancy and racial and ethnic differences, but the reality is, too many American teens are still having babies,” CDC Director Tom Frieden said in a statement.”By better understanding the many factors that contribute to teen pregnancy we can better design, implement, evaluate, and improve prevention interventions and further reduce disparities.”