Random quotes: Marty Weitzman


On occasion, I like to take things from my shelf, open them up and see what I find. I did this last month. For this month, I picked up the May 1998 issue of the Quarterly Journal of Economics and flipped open to page 380. Read on, this is a good one … 

Here is what I found:

Station Sci-Fi is an agricultural-research-space-station colony that is self-contained in a translucent biosphere currently orbiting Jupiter. The state of technology on Station Sci-Fe, and therefore its productivity, is in any period proportional to the number of different productivity-enhancing “idea-cultivars” that already exist at the beginning of that period. Productive capacity determiens the total amount of effective sci-fi automated greenhouse services, which can be used either to grow food for the colony’s consumption or to raise yet more idea-cultivars. New idea-cultivars are obtained by cross-pollinating existing uncrossed idea-cultivar pairs and raising the resulting fertaile seeds to maturity in the sci-fi automated greenhouses dedicated to this purpose.

For non-economists reading this, let me tell you that this is not something you read everyday in economics articles! Believe it or not, it was designed to make an abstract model have a more realistic flavour. This is from an paper, “Recombinant Growth” by Marty Weitzman (a Harvard professor). Suffiice it to say, he is somewhat ahead of the curve on application.

He goes on:

Time on Station Sci-Fi is measured in periods of duration equal to the natural development cycle of an idea-cultivar.

This leads to a “Simple Economy Theorem” the proof of which is omitted from the paper “in the interest of brevity.” So what this says to me is that the QJE decided it was more useful to have a floral description of a hypothetical economy than the proof of the main theorem of the paper!

By the way, the intuition behind the proof is given some space:

The simple sci-fi economy model behind equation (21) was created to exhibit the basic theme of this paper in its most stark form. The limiting potential to generate new ideas here increases quadratically in ideas while the capacity to process new ideas increases linearly; the ultimate constraint on economic expansion is linear.

Oh yeah, and equation (21) says that the growth of the stock of knowledge on Station Sci-Fi is equal to s*B where s is the fraction of the sci-fi automated greenhouses not dedicated to producing the colony’s food and B is the coefficient of productivity enhancement per idea-cultivar. Bear in mind that this is again similar to my honours thesis (described here). I can’t believe I didn’t think of this science fiction metaphor in expositing it and, instead, stuck to the corn field; the traditional parable of economic growth theory.

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