I've several times seen people refer to studies showing that abstinence-only sex ed programs don't work. What they mean by that is that people who go through the abstinence-only programs aren't any more likely than those who go through comprehensive programs to have had unprotected sex. If the goal is to prevent sexually-transmitted diseases and unwanted pregnancies by encouraging people not ready for parenthood not to engage in sex at all, it seems not to work. I didn't look closely at any of these studies, just accepting that they were correct, because I've never favored only telling people to abstain. There's nothing wrong with providing information about condoms and hormonal methods of contraception. In fact, without providing the full information, some might never realize the failure rates of various methods of contraception and those that do choose to use them might not do so properly, thus trusting increasing the unreliability of something they rely on. So it's counterproductive for those who want to reduce sexual activity even apart from pregnancy and STDs to resist presenting comprehensive information.
Someone (I don't remember who) recently directed me to this study. I haven't checked any other studies as closel, but I checked Wikipedia for any further long-term studies on this, and I didn't find anything but this study and a study on a different topic about how some abstinence-only programs didn't do it right (i.e. they had some false information in their educational package). If this data is correct then those who have been posturing about abstinence-only programs not working have been spinning the science with as much ideologically-motivated one-sidedness as such people regularly accused the Bush Administration of doing, not exactly the best behavior among those accusing others of being anti-science for doing the same thing.
A lot of criticism of abstinence-only sex ed has been that it's lack of information about contraception leaves kids with the wrong information, thus making it more likely that they won't use proper precautions if they do have sex. This turns out to be disconfirmed. The kids who went through the abstinence-only programs were as well-informed on such matters as the kids in the control group, and they didn't have any higher rate of unprotected sex than anyone else. It may well be that comprehensive sex ed would have led to their being more informed than average, but it's not as if abstinence-only sex ed made them less-informed, as many opponents of abstinence-only have been claiming. Given this study, it seems that it's just as much anti-science to call abstinence-only education dangerous and even a cause of unwanted pregnancy and the spread of STDs as it is to promote abstinence-only education as the best method of preventing STDs and unwanted pregnancy. Such behavior is irresponsible and pretty obviously motivated by ideology while at odds with the facts, the very thing the Bush Administration has repeatedly been accused of doing on this issue.
When you look at the fine print, you can also see that this is looking at the long-term effects of early abstinence-only programs that aren't continued in high school,and according to this story they did find an initial effect of delaying the first sexual encounter that dropped off in later years, the same later years that these kids weren't continuing to receive abstinence-only sex ed. Isn't that a bit suspicious if the conclusion is supposed to be that abstinence-only sex ed doesn't work? It's not clear that this study really shows what it's been taken to show, which is that abstinence-only sex ed doesn't work.
Keep in mind also that there is a study that shows that a number of abstinence-only programs had curriicula that included falsehoods and questionable elements. So if you examine just the actual abstinence-only programs, it doesn't necessarily tell you what would happen if it were done with more care to present the correct information. Even if some of the false information might have led some to be more likely to be abstinent, it may have gone the other way with some, especially those who know the information being presented is false, which could incline some to reject everything that's being said as a result, including the abstinence message and the correct rates of failure of condoms or other contraceptive or STD-preventative measures. Remember that we're talking about teenagers here. Also, some of these programs were determined to be teaching religious doctrine. I have no idea what that means, and I'm certainly aware that some things claimed by some to be religious doctrine simply aren't, e.g, that life begins at conception, while others are, e.g. that it's morally wrong to engage in sex outside marriage (although I think a secular argument exists for such a view). But the point is that some might turn off to the whole enterprise if their view is that this is religious education.
So what they study does seem to show is that earlier abstinence-only sex ed, as it's actually been taught (as opposed to how it should be taught), doesn't seem to affect later sexual behavior if that kind of sex ed doesn't continue into high school, but it doesn't tell us anything about what happens if it does continue, and the fact that some of these programs were presenting false information might skew the results in either direction. It may well be that comprehensive sex ed would do better on the measure that we're discussing, but this study doesn't help us know that, and I know of none that do. I do see some that show increased effectiveness among those receiving comprehensive sex ed over control groups, but until we have a long-term study that actually looks at those who receive abstinence-only sex ed in high school, the facts simply aren't fully available on that question, and it would ideally help if someone could conduct a study on the best abstinence-only programs compared to abstinence-only programs as they actually occur, to see if there's any difference.