An experimental abstinence-only program without a moralistic tone can delay young teens from having sex, a new study found.

Billed as the first rigorous research to show long-term success with an abstinence-only approach, the study released Monday differed from traditional programs that have lost U.S. federal and state support in recent years.

The classes didn't preach saving sex until marriage or disparage condom use. Instead, they involved assignments to help students around the age of 12 see the drawbacks to sexual activity at their age. It included having them list the pros and cons themselves, and it found their "cons" far outnumbered the "pros."

The study appears in the February edition of Archives of Pediatrics & Adolescent Medicine. It was funded by the National Institute of Mental Health and involved 662 black children in Philadelphia.

The students were assigned to one of four options: eight hour-long abstinence-only classes; safe-sex classes; classes incorporating both approaches; or classes in general healthy behavior. Results for the first three classes were compared with the group that had only the general health classes. That was the "control group" the study used for comparison.

Two years later, about one-third of abstinence-only students said they'd had sex since the classes ended, versus nearly half — about 49 percent — of the control group. Sexual activity rates in the other two groups didn't differ from the control group.

Valerie Huber, executive director of the National Abstinence Education Program, praised the study and said she hopes it revives government interest in abstinence-only sex education.

Critics of abstinence-only programs have long argued that most evidence shows they don't work. The new study challenges that, but even the authors say the results don't mean that more comprehensive sex education should be ignored.

The abstinence-only program was based on social psychology theories about what motivates behavior. It encouraged abstinence as a way to prevent pregnancy and sexually transmitted diseases.

Psychologist John Jemmott III, the lead author, called the findings surprising given negative results in previous abstinence-only research. Jemmott said the single focus may have been better at encouraging abstinence than the other approaches in his study.

"The message was not mixed with any other messages," said Jemmott, a professor at the University of Pennsylvania who has long studied ways to reduce risky behavior among inner-city youngsters.

Monica Rodriguez of the Sexuality Information and Education Council of the United States, an advocacy group favoring comprehensive sex education, said the study doesn't mean other abstinence-only programs would work.

"It's unfair to compare this abstinence-only intervention to the typical abstinence-only-until-marriage program that young people in this country have been put through," she said. These typically portray sex and condom use in a more negative light, she said.

Rodriguez said the program studied might be one approach to try with younger children, but that it probably would be less successful with older, more sexually experienced teens.

Almost one-fourth of the teens studied said they'd already had sex at least once, similar to other studies of urban, mostly black children of middle school age, around 11 to 13.