Our goal, then, should be to drive the price of condoms below zero, by rewarding people who use them. In other words, we should pay a bounty for used condoms. The best of all possible bounties would be one that is more valuable to abstemious Martins than to promiscuous Maxwells. With that in mind, the journalist Oliver Morton has made the marvelous suggestion that if at least some abstemiousness is due to shyness and the inability to find partners (while the promiscuous have relatively little trouble in this regard), then the answer might be to establish a government-funded dating service: bring us a used condom and we'll get you a date.

The entire problem - along with the entire case for subsidies - would vanish if our sexual pasts could somehow be made visible, so that future partners could reward past prudence and thereby provide appropriate incentives. Perhaps technology can ultimately make that solution feasible. (I imagine the pornography of the future: "Her skirt slid to the floor and his gaze came to rest on her thigh, where the imbedded monitor read 'This site has been accessed 314 times.'")

Or, as one of my Slate readers suggested, we could have an online service to record negative HIV test results. You'd type in your prospective partner's name and get a response like "Last negative test result 7/4/2006." Or, to protect privacy, you'd type in not a name but an ID number provided by the partner. Your screen could show both a test result and a photo to avoid fake IDs. This strikes me as such a good idea that I can't figure out why nobody's doing it yet. Until then, the best we can probably do is to make condoms inexpensive - and get ride of those subway ads.

Addendum

In 1996, Slate magazine published an abbreviated version of this chapter that generated hundreds of email responses. Quite a few of those responses were both thoughtful and interesting, and helped me to improve the presentation you've just read. Others contained nothing but a line or two of invective. To those, I usually responded with a short note that read "I'm sorry, but from the email you sent me I was not able to ascertain at exactly which point you stopped following the argument. If you can be more precise about where you got lost, I'll do my best to make it clearer." In a remarkable number of cases, I got responses that were both thoughtful and apologetic, and a few of those led to multiround correspondences that taught me something.

Other readers seemed bound and determined to miss the point by miles. One, brandishing his credentials as a medical doctor, termed the column "particularly unfortunate" and - in a letter that was published in a subsequent issue of Slate - explained why:

"We are at a stage in the HIV epidemic in which heterosexual spread is becoming increasingly significant. Casual readers...may justify increasing their sexual-risk-taking behavior. Unfortunately, failure, lasting in a shortened lifetime, can result from a sexually successful one-night stand.

For an appropriate sequel, the editor of Slate might solicit an article...defending Russian roulette as statistically OK but cautioning that three loaded chambers is too risky."

One of the great discoveries of 19th century economics was the principle of comparative advantage, according to which people are most successful when they stick to the things they're good at. (It's actually quite a bit subtler than that, but this oversimplified version suffices for the application I'm about to make.) The principle of comparative advantage explains why some people become medical doctors, while other, different, people go into fields (such as economics) that require at least a minimal ability to reason logically.