Tuesday, September 25, 2007

Infectious disease, how bad does it do a body?   posted by Razib @ 9/25/2007 11:54:00 PM

In my post below I respond to Bryan Caplan's critique of Greg Clark's claim that disease can increase per capita income because it reduces population (i.e., same population has a bigger resource base to work with).1 I go the route of the two handed economist by suggesting that whether Clark or Caplan is right depends on the details.2 Herrick adds in the comments:
Caplan's big claim is that almost anything that persistently raises death rates is likely to persistently reduce output per living worker. It that true?

One possible source of persistent increases in death rates that have no impact on productivity: Many kinds of infectious disease.

I'd welcome medically-informed comments on the topic, but it seems possible for infectious disease (from, say the bad sanitation that Clark emphasizes) to raise the chance of dying any given month without appreciably hurting your productivity most of the time.

Scenario: You get sick for a week or two every couple of years, and if you survive, you go back to being productive. If you don't survive, well then, you're pushing up the death rate.

As I suggest below I think that Caplan is wrong if he wants to claim that productivity is always decreased in direct proportion to the increased disease load (ergo, death rate) of a population. This would prevent the rise in incomes which Clark predicts as the lower productivity of each individual means that the same amount of land can support fewer people at or above subsistence. In A Farewell to Alms Clark reports a rise in incomes after the Black Death, and, amongst native peoples in the New World after Old World diseases ravaged them. Obviously this is one extreme cause: a highly lethal infectious disease which cuts down a large proportion of the population very quickly, and then recedes. The other scenario is a case where there is an endemic infection which reduces physiological fitness across the whole population, reducing lifespan and increasing death rates, but also dampening economic productivity. Then there are cases where there is a wide variance within the population in regards to susceptibility toward infectious agents. This might be more like the first scenario, a large number of people die very quickly, while many others are spared because of some immunity. And so on.

From Darwinian first principles it seems that there should be a large number of pathogens which are infectious but not fatal. Though reducing physiological fitness, they don't knock out their host because to do so would result in their own reduced evolutionary fitness. But hey, Herrick asked for expert opinion. I was actually hoping that someone with medical expertise (e.g., tropical diseases?) would weigh in on that thread, but that didn't happen. So I come to you with open hands and ask you to enlighten....

Update: Greg Clark responds directly to the Caplan critique. As a non-economist I'm more interested in what the empirical historical data says, and what little I know seems to agree with the general thrust of Clark's point.

1 - That sentence should filter out chimpanzee readers since it should be totally incomprehensible to them.

2 - No shit it depends on the details!