NASHVILLE, Tenn. - Like many parents, Mary Donaldson dreaded having "the talk" with her teenage daughter, Brooke.

That's why Donaldson turned to the True Love Waits program at Tulip Grove Baptist Church in Old Hickory, Tenn. The program promotes abstinence before marriage and urges parents to talk with their kids about sex.

Brooke made a "purity pledge" three years ago.

Abstinence-based sex education programs, backed by religious conservatives, have been criticized as ineffective and based more on religion than scientific reality. But a new study by the RAND Corp. finds that purity pledges are surprisingly effective in delaying the onset of sexual activity.

Students ages 12 to 17 who made purity pledges were 21 percent less likely to have sex within three years of making a pledge than were teens who wanted to avoid sex before marriage but did not make a pledge, according to RAND's research, published online by the Journal of Adolescent Health.

The RAND study focused specifically on teens who said they wanted to delay having sex, and who had parents and peers supporting that decision. The study excluded teens who were not trying to abstain from premarital sex.

About a quarter of the 2,000 teens surveyed had made a virginity pledge of some kind. Without a pledge, 42.4 percent of that group would have sex within three years. With a pledge, that figure dropped to 33.6 percent.

Groups like Nashville-based True Love Waits have popularized purity pledges since the 1990s. The program, run by Southern Baptist publisher LifeWay, claims that more than 1 million teens have signed purity commitment cards.

Those cards read: "Believing that true love waits, I make a commitment to God, myself, my family, my friends, my future mate, and my future children to be sexually abstinent from this day until the day I enter a biblical marriage relationship."

RAND researcher Steven Martino says purity pledges work a little bit like Weight Watchers.

"It's public accountability," he said. "They declare their intentions and have peer support to hold them accountable."

But he warned that pledges wouldn't work for everyone.

"Virginity pledges cannot substitute for a comprehensive program of sex education," Martino said. "Most teens do have sex, and those teens need to know how to protect themselves against unwanted consequences."

Martino said pledgers were just as likely as nonpledgers to use condoms when they have sex.

A critic of abstinence-only approaches to sex education, Frank Boehm, professor of obstetrics and gynecology at Vanderbilt University Medical Center, says purity pledges have a place in preventing teen sex.

"There are benefits to not having sexual intercourse when you are 15, 16 or 17," he said. Along with risking disease and pregnancy, many teens are not emotionally prepared for sex, Boehm said.

Richard Ross, who co-founded True Love Waits in 1993 while he was youth pastor at Tulip Grove Baptist, believes the program works because of its holistic approach. Before making a pledge, he said, students go through a series of classes, and talk with their parents. After making a pledge in front of their churches, students meet in ongoing support groups to sustain their commitment.

Without that support, the pledges are ineffective, said Ross, now professor of student ministry at Southwestern Baptist Theological Seminary.

"Students are not making a pledge to a program," he said. "They are making a promise to God."

At Tulip Grove Baptist, about a dozen students take a purity pledge each year, says Alec Cort, the church's youth pastor. The church holds an event around Valentine's Day each year. After the child makes a pledge, parents give them a silver ring.

Mary Donaldson's daughter, Brooke, said that her parents supported her decision to make a pledge but didn't push her.

"I don't have any pressure," she said. "I know what my parents expect of me, and I try to do the best I can. If I fail, I'm not going to be kicked out of the house or anything."