Let's be realistic, say critics of President Bush's proposal to spend $135 million next year on "abstinence only" sex education: Kids are gonna be kids.

So it would be "dangerous and unnecessary" to increase spending on abstinence programs, Rep. James Greenwood, R-Pa., wrote in a letter to the president. "There is no scientific evidence that 'abstinence only until marriage' programs work."

But there is. In fact, my Heritage Foundation colleague Robert Rector has found that "abstinence only" programs have a record of success that the "if you're gonna do it, do it safely" programs can't match.

One "abstinence only" program, for example, calls on teenagers to take a "virginity pledge." Researchers used a sample of more than 5,000 students to evaluate it for the American Journal of Sociology.

They found that taking the pledge reduces by one-third the probability that an adolescent will begin sexual activity. Pair the pledge with strong parental disapproval of pre-marital sex, and the probability that teens will become sexually active drops by 75 percent or more.

Another program, called "Not Me, Not Now," used radio and TV ads to promote abstinence among teenagers in Monroe County, N.Y. A study published in the Journal of Health Communications found that during the period the ads were being aired, the pregnancy rate for girls aged 15 to 17 fell from 63.4 pregnancies per 1,000 girls to 49.5 pregnancies. The sexual activity rate of 15-year-olds across the county, meanwhile, dropped from 46.6 percent to 31.6 percent.

Then there's the Abstinence by Choice program, which operates in 20 schools in the Little Rock, Ark. area. It targets seventh-, eighth- and ninth-grade students and reaches about 4,000 youths each year. A researcher at Case Western Reserve University's School of Medicine, using a sample of nearly 1,000 students, found the program reduced the sexual activity rates of girls by about 40 percent and the rate for boys by approximately 30 percent, compared with similar students who weren't in the program.

Other "abstinence only" programs, from Operation Keepsake in Cleveland to the Teen Aid Family Life Education Project in California and five other states, show similar results. Research conducted by the Alan Guttmacher Institute found that the Postponing Sexual Involvement program in Atlanta reduced sexual initiation rates among eighth-grade boys by 60 percent and among eighth-grade girls by 95 percent.

So-called "safe sex" programs, by contrast, can't boast such a success rate. But considering that most are little more than thinly disguised efforts to promote condom use, how could they? Programs that carry the caveat "if you have sex, here's how to do it" undermine warnings against pre-marital sex. Small wonder that many such programs are now called "abstinence-based" or "abstinence-plus," when they usually include only a casual-and often winking-nod to the idea of avoiding sex.

These programs send a clear message that society expects, and accepts, early sexual activity. Guidelines developed by the Sexuality Information and Education Council of the United States (SIECUS), for example, recommend teaching children as young as 5 about masturbation, teaching nine-year-olds about oral sex, and teen-agers about anal intercourse. (Hard to believe, I know, but it's true.)

This despite the fact that sexually transmitted diseases (STDs) in this country have reached epidemic proportions among young people. Some 3 million teen-agers contract STDs each year, afflicting roughly one out of every four teens who are sexually active.

True abstinence programs help young people build an understanding of commitment, fidelity and intimacy that will serve as the foundations of healthy marital life. Can the same be said for classroom demonstrations involving condoms and cucumbers?

Edwin J. Feulner, Ph.D. is president of The Heritage Foundation, a Washington-based public policy research institute.