POSITIVE NONPECUNIARY EXTERNALITIES. Economics suggests prudes should want to have fun.

The "More Sex" thesis: If prudes occasionally slept with strangers, it would slow the spread of STDs.

Here's how it works. One such prude walks into a bar, and he's uninfected. If he takes home an uninfected woman, great -- he distracted her from a potential disease carrier. If he gets herpes, that's also great, because he's sexually conservative and won't pass the infection along very often. Better him than someone with less self control.

Either way, society benefits when the chaste open up slightly. "Slightly" is key, because too much "openness" spreads more disease than it diverts. After studying AIDS in England, Harvard's Michael Kremer put the cutoff at 2.25 partners per year.

From here Mr. Landsburg introduces the concept that ties virtually all these essays together -- people should feel their actions' effects. A sexual conservative considers the harm to himself, but not the benefits to others, of catching a disease. One could call it a behavior-consequence gap.

The illustration, (via Andrew Sullivan) from Steven Landsburg's new More Sex is Safer Sex: The Unconventional Wisdom of Economics, doesn't really come as a surprise. There's a paper, I believe it's Buchanan and Stubblebine's "Externality," that illustrates the optimality of some people not receiving immunization shots, from which the basic argument follows. Another observation has the possibility for greater controversy.

[Professor Landsburg] also discusses the jury system at length. He's right that juries should hear all the available evidence, including past convictions: The government already trusts juries to sort out pertinent from irrelevant facts, so more information can't hurt. If juries can't put a defendant's (or even accuser's) history in perspective, we shouldn't have juries.

The problem, dear reader, as mathematician John Allen Paulos has argued, is that juries have difficulty with conditional statements. (Test yourself: which of the two following statements is more likely to be true? (A) Judy is a bank teller. (B) Judy is a bank teller and a feminist.) In trials, the mishandling of conditional statements matter. Consider (A) The defendant battered his wife. (B) Most spouse batterers do not murder their wives. Will the jury be more likely to come to the correct conclusion if they are also told (C) Most murdered wives have been battered?

I will read the book sometime in the future and have a review ready.