Log Utility From Geometric Risk Neutrality
Published in Decision-Making in Economics eJournal (SSRN), 2026
In multiplicative wealth dynamics, “equal and opposite” moves correspond to multiplying and dividing by the same factor, not adding and subtracting equal amounts. We show that if an agent is geometrically risk-neutral in the sense that at any wealth level \(w\), they are indifferent between keeping \(w\) and taking a zero time-average growth bet, then their utility must be affine in \(\ln (w)\), hence \(u(w) = \alpha \ln (w) + \beta\).
