Maybe twice a year I go looking for this comment and can’t find it, so I’m copying it into shortform:
Oh, I can just give you a class of nontrivial predictions of expected utility theory. I have not seen any empirical results on whether these actually hold, so consider them advance predictions.
So, a bacteria needs a handful of different metabolic resources—most obviously energy (i.e. ATP), but also amino acids, membrane lipids, etc. And often bacteria can produce some metabolic resources via multiple different paths, including cyclical paths—e.g. it’s useful to be able to turn A into B but also B into A, because sometimes the environment will have lots of B and other times it will have lots of A. Now, there’s the obvious prediction that the bacteria won’t waste energy turning B into A and then back into B again—i.e. it will suppress one of those two pathways (assuming the cycle is energy-burning), depending on which metabolite is more abundant. Utility generalizes this idea to arbitrarily many reactions and products, and predicts that at any given time we can assign some (non-unique) “values” to each metabolite (including energy carriers), such that any reaction whose reactants have more total “value” than its products is suppressed (or at least not catalyzed; the cell doesn’t really have good ways to suppress spontaneous reactions other than putting things in separate compartments).
Of course in practice this will be an approximation, and there may be occasional exceptions where the cell is doing something the model doesn’t capture. If we were to do this sort of analysis in a signalling network rather than a metabolic network, for instance, there would likely be many exceptions: cells sometimes burn energy to maintain a concentration at a specific level, or to respond quickly to changes, and this particular model doesn’t capture the “value” of information-content in signals; we’d have to extend our value-function in order for the utility framework to capture that. But for metabolic networks, I expect that to mostly not be an issue.
That’s really just utility theory; expected utility theory would involve an organism storing some resources over time (like e.g. fat). Then we’d expect to be able to assign “values” such that the relative “value” assigned to stored resources which are not currently used is a weighted sum of the “values” assigned to those resources in different possible future environments (of the sort the organism might find itself in after something like its current environment, in the ancestral world), and the weights in the sums should be consistent. (This is a less-fleshed-out prediction than the other one, but hopefully it’s enough of a sketch to give the idea.)
Of course, if we understand expected utility theory deeply, then these predictions are quite trivial; they’re just saying that organisms won’t make pareto-suboptimal use of their resources! It’s one of those predictions where, if it’s false, then we’ve probably discovered something interesting—most likely some place where an organism is spending resources to do something useful which we haven’t understood yet. [EDIT-TO-ADD: This is itself intended as a falsifiable prediction—if we go look at an anomaly and don’t find any unaccounted-for phenomenon, then that’s a very big strike against expected utility theory.] And that’s the really cool prediction here: it gives us a tool to uncover unknown-unknowns in our understanding of a cell’s behavior.
Maybe twice a year I go looking for this comment and can’t find it, so I’m copying it into shortform: