Several times -- when I have observed that I know from my own experience that if one simply does not have sex until getting married, one will in the normal course of things successfully avoid STDs and unplanned pregnancies -- I have had quoted to me the "well known fact" that abstinence only sex education does not work. A quick browse around for studies showed several solid ones, including one commissioned by congress and the Department of Health and Human Services. These all showed that rates of sexual activity, pregnancy and STD infection were not decreased by "abstinence only" sex ed programs.
This didn't really seem like a surprise to me. That a school makes kids sit through several years worth of classes which emphasize that they should wait until marriage to have sex does not by any means mean that the students themselves will resolve to follow that course of action. Heck, schools try hard to teach history, reading, math and science, and students often don't absorb those lessons either -- despite the fact that there is nothing but upside to knowing about those subjects. One must assume that students will be even more unlikely to absorb lessons about abstinence -- since that involves "not getting any" and being something of a social oddity.
I and a number of my peers avoided sex until marriage, but that's because within our particular sub-culture of orthodox Catholics there was a strong reason not to -- something called "mortal sin" that we all believed in. If I had not believed that it was a mortal sin to have sex before marriage, and belong to a peer group that strongly supported that belief, it seems highly unlikely that I would have followed that course. MrsDarwin and I had been dating for nearly four years by the time we were finally able to get married. If we hadn't had a very, very strong motivation to wait -- then why do it?
So I'd essentially figured that the reason why abstinence education "didn't work" was because most of the kids were just sitting there in class thinking "this is stupid" and didn't make any particular effort to be abstinent.
I continue to think I'm right on that, but I ran across a post by Megan McArdle who pointed out something that I hadn't noticed: The most comprehensive recent controlled study to date, the one commissioned by the DHHS, found that groups of student put through several years of one of four commonly used abstinence-only programs did not abstain from sex (or get pregnant or get STDs) at different rates from those in the "control" groups. In two of the studied school districts, the control group was receiving only a "health and science" class which provided little to no information on contraception and STDs. In the other two districts, the control group received a fairly comprehensive sex ed program centering on contraception.
In all four districts, there was no significant difference between the students getting abstinence-only education and those in the control group.
What that means is not just that "abstinence-only sex ed doesn't work", it means that neither form of sex education significantly changes the way teenagers behave. There might as well be no sex education at all -- which to my mind would be just fine. This is one of those areas in which any particular approach to teaching on the topic is going to go against the sensibilities of at least some parents. And results like these only serve to underline that there are some areas which public schools (or indeed schools in general) simply don't have much ability to change behavior. This is one of for the parents and culture, not the schools.