Milton Friedman and the surprising rationality of trees
By: Oliver Beige
Tags: Evolution, Forest, Economics, Milton Friedman, Tree

Milton Friedman and the surprising rationality of trees

In one of the most famous essays of 20th century economics, one of its most famous protagonists offers an astounding hypothesis on tree growth, just to dismiss it out of hand. Indeed, the dismissal is understood to be mutual between author and reader, as it sets up the author’s intellectual tour de force. Oliver Beige Dec 25, 2020·6 min read

Even if the premise is wrong, the conclusion can still be correct as long as it is congruent with the empirical findings. There is no such thing as a false assumption, even if it is as outlandish as positing that trees are rational actors. There are only empirical discrepancies between prediction and reality.

The famous author is Milton Friedman, the essay is his famous The Methodology of Positive Economics, and his readership includes some 15,000 scholars who cite the essay, mostly approvingly. It changed modern economics.

Consider the density of leaves around a tree. I suggest the hypothesis that the leaves are positioned as if each leaf deliberately sought to maximize the amount of sunlight it receives, given the position of its neighbors, as if it knew the physical laws determining the amount of sunlight that would be received in various positions and could move rapidly or instantaneously from any one position to any other desired and unoccupied position. Now some of the more obvious implications of this hypothesis are clearly consistent with experience: for example, leaves are in general denser on the south than on the north side of trees but, as the hypothesis implies, less so or not at all on the northern slope of a hill or when the south side of the trees is shaded in some other way. Is the hypothesis rendered unacceptable or invalid because, so far as we know, leaves do not “deliberate” or consciously “seek,” have not been to school and learned the relevant laws of science or the mathematics required to calculate the “optimum” position, and cannot move from position to position? — Milton Friedman, The Methodology of Positive Economics, 1953.

Tree vs human. Photo by Gilly Stewart on Unsplash

But do we actually know that trees are not rational actors in Friedman’s sense? Apart from accepting that trees didn’t go to school and didn’t learn calculus there, the observed behavior of grown trees matches his hypotheses remarkably well.

So we shouldn’t quite dismiss such a theory out of hand. And as it turns out, the literature in forestry about tree growth is quite voluminous, even if admittedly mostly younger than Friedman’s 1953 essay.

Circumnutation and trees as deliberate seekers

One of the basic laws of operations research is that in any optimization program, corner solutions are rarely optimal and interior solutions are often hard to locate.

This basic law has its equivalent in economics in Rudi Dornbusch’s overshooting model, which posits that exchange rate volatility is often driven by overeager attempts to reach such an interior market-clearing equilibrium.

Even human behavior outside the economic realm often exhibits this kind of circling around the equilibrium point, as mobility researcher have known for some time. Drivers don’t hold an “optimum” distance to the preceding vehicle, they cycle within a posited distance window.

Trees are rooted, so they don’t quite have the same range of motion as a human to optimize distance. But somewhat surprisingly they use whatever range they have to optimize growth in a process called circumnutation.

Circumnutation, the observable rotating motion of a growing single-stem plant, is still not fully understood, but its analysis goes back to a scholar even more famous than Friedman: Charles Darwin, who prominently described it in his penultimate book The Power of Movement in Plants.