Middle-aged politicians who promote sexual abstinence among teens run a great risk of derision.

That's what happened to Texas Gov. George W. Bush last month when he gave a speech in South Carolina arguing that "we must encourage the children to make the choice of abstinence." The front-runner for the Republican presidential nomination urged that "we spend as much each year promoting the conscience of our children as we do on providing them with contraception."

Bush was also critical of programs against teen pregnancy that promoted contraception. "It seems like to me the contraceptive message sends a contradictory message," he declared. "It tends to undermine the message of abstinence."

The commentariat had fun with Bush on this one. The Sacramento Bee ran this puckish headline: "Wild Oats Sown, Bush Counsels Abstinence." Bush was said to be pandering to the Christian right and unrealistic about the forces of nature. A pandering prude is not the thing to be these days.

The dart-throwers suggested that programs promoting abstinence don't work in preventing teen pregnancy. Many cited the nation's expert on the subject, Doug Kirby.

Now, Kirby, a senior research scientist at ETR Associates in Scotts Valley, Calif., is a very careful analyst. He'll tell you evaluations of abstinence-only programs find no proof that they reduce teen pregnancy.

But he'll also tell you this is so because there are few studies of abstinence-only programs. The one really good study, involving a California program, found it had no impact on teen pregnancy. Four other less rigorous studies reached the same conclusion.

That, however, doesn't make a definitive case against them. "Abstinence-only programs are a very heterogeneous group of programs," he says from his California office, "and it's not fair to judge them by these studies. We just don't know."

Kirby will tell you flatly that on the basis of a lot of good research, Bush is wrong in saying contraception programs encourage teen sex. "We have solid evidence that they do not," he says.

But his social science intuition also tells him this: "Probably some abstinence-only programs are effective, and some are not."

Public arguments about the sexual education of teens tend to mix up the issues of what are appropriate moral lessons to give them and what works best in preventing teens from getting pregnant.

If what you care about is teen pregnancy, the data suggest the programs that work best combine abstinence messages with contraception as a backup. "What a large majority of American sexuality educators and a large majority of Americans are pushing for is abstinence plus," Kirby says.

This means "you give real weight to abstinence, you give it serious attention, you say that abstinence is the only method that is 100 percent effective against pregnancy and sexually transmitted diseases. But then you also talk about condoms and contraception in a balanced and accurate manner."

The age and sexual experience of the teens involved also matters. "If you only have 5 or 10 percent of the youth in a grade level who have had sex, then the abstinence-only message is fine, it's appropriate," he says. "When a larger percentage have had sex, I think we have a moral obligation to give them accurate information about condoms and contraception."

Bush doesn't deserve all the grief he's gotten over teen abstinence programs. There's a practical as well as a moral case for them. The teen birthrate has been coming down because of both a decline in teen sexual activity and better birth control.

But Bush has to be clearer if he's to avoid suspicions that he's just playing interest-group politics by tossing a few extra bucks to abstinence programs. If he has moral objections to all contraception education, he should say so -- and face the political consequences. If he doesn't, he should join the abstinence-plus forces.

The sex education wars are difficult because they are battles over the public messages kids should be sent about sexuality. For parents, this is a deeply personal matter. Nobody should be dismissive or disrespectful when significant groups of parents regard messages sent in sex ed as too permissive.

"It's important that parents have a right to be informed about a program and to withdraw their children if they wish to," says Kirby. But he also argues that the number of kids born to kids can be reduced by contraception programs. This is one divisive moral issue on which the country is groping toward consensus. If he chose to, Bush could help us get there.