So I've just found out that sexual contacts in human beings tend to follow power laws. Apparently, this has been known since the first result measuring the network of sexual contacts appeared in 2001.
It seems to me that the implications of this finding for questions of sexual education in schools are huge.
Basically, what the research says is that a small percentage of individuals have an extremely large number of sexual partners, thus resulting in a small world phenomenon: the distance between any two people (i.e. the number of sexual partners one must go through to reach one person starting from the other) is not too large because one can always go through some of these highly connected hubs.
If you don't understand this, then think of blogs - the number of sexual partners behaves like blog traffic, with a small number of users with extremely high traffic (Instapundit, Daily Kos, etc).
If this is so, then its clear that teaching sexual abstinence in school is going to have very little effect in preventing hiv. Indeed, even assuming that teaching abstinence is actually effective - still, convincing people with a relatively small number of sexual partners to avoid sex altogether will have little effect as the connectivity of the network follows from the hubs of people with extremely high sexual activity. The only way to effectively stop the transmission of hiv would be at those hubs.
Now ask yourself what is more likely to appeal to someone who has a huge number of irregular sexual partners: an appeal to give up sex altogether or an appeal to properly use condoms?