"In other fields do you see people spouting 'I just thought of this, and I'm an economist, so it must be true!'"

Well, in other fields they don't say "I'm an economist" because they're not economists, but we know exactly what you mean there.

That's a comment in a Metafilter thread about that book "More Sex Is Safer Sex." Someone else says, "Ah. I see the new trend in economics, post Freakonomics, is to say something outrageous to get people's attention and then try to back it up with numbers."

We talked about the "More Sex" book last Friday, and a really cool thing was that the author, Steven E. Landsburg, participated in the comments and got into a whole back-and-forth with people here. Here's the exchange he had with me. I said:

The phrase is "more sex is safer sex," not "more sex is more sex and assuming sex is generally good, there are more total benefits weighed against the costs of disease, depending on just how relatively bad you think it is to have HIV." I still don't see how more sex is safer sex. But it is a catchy phrase. I'll give you that. It made me blog about it.

He said:

I hope you will read the book, or at least the chapter; if you do, you'll find that there are several related points, and I am careful to distinguish among them.

The first point is that under quite reasonable assumptions drawn from standard epidemiological models, more sex probably IS safer sex; i.e. it actually reduces the spread of STDs. That conclusion relies on some modeling assumptions, but the assumptions are nowhere near as naive or silly as many of the commentators in this thread seem to assume.

The second point is that REGARDLESS of your modeling assumptions, and REGARDLESS of whether more sex actually reduces STDs, more sex (by the relatively chaste) MUST increase the excess of benefits over costs, not as a matter of idle speculation but as the conclusion of a careful logical argument.

And the third point is that following logical arguments is both illuminating and fun. Far more illuminating and far more fun than constructing and attacking straw men, in fact.

I said:

I don't see why this assertion is susceptible to logical analysis -- careful or not. Isn't it just [an] article of faith that sex with a partner produces happiness?

He said:

The assumption is that on average, sex produces the expected amount of happiness. Sometimes much more than expected, sometimes much less than expected, but not systematically one or the other. The justification for that assumption is that if people are systematically wrong in a predictable direction, they'll eventually learn and correct for that.

That turns out to be all you need to get the analysis going (and if you won't grant me this much of a start, do be aware that the same assumption underlies 90% of economic analysis, and, stereotypes to the contrary, economic analysis does yield lots and lots of accurate predictions).

Now you can reason as follows (this is highly stylized, but it gives the flavor): You take a new partner when the expected benefits to you exceed the expected costs and not otherwise. Sometimes that will cause you to turn away partners even when the total expected benefits (to you and other members of society) do exceed the expected costs (because you count only some of the benefits, not all of them). And because, on average, expected benefits equal actual benefits, it turns out that actual benefits-minus-costs would be higher if you took additional partners.

(The ways in which your taking an additional partner benefits your neighbors are, incidentally, more substantial---and more subtle---than those mentioned in the New York Times review.)

I quoted him -- "The justification for that assumption is that if people are systematically wrong in a predictable direction, they'll eventually learn and correct for that" -- and got sarcastic:

I guess I'm cynical. I stopped believing in fairy tale romance long ago.

Lots more at the Friday post. I really loved that Landsburg came over and duked it out in the comments. Let's see more of that sort of thing.

IN THE COMMENTS: Theo says:

Yes! That was one of the best threads in recent memory. It was a great topic. And it was interesting, logically challenging, and pound-on-the-table funny by turns—yet remarkably free of slash-and-gash trolldom.

You know, I've been grousing about trolls, the quality of comment writing, etc. here and elsewhere. I know at least a few longtime bloggers and commenters who seem depressed and generally bummed out by the blogosphere. But this is the kind of thread that restores your faith in the whole enterprise.

Hate to turn all Polyannaish on everybody.