Gatheral's Conjecture Revisited

Abstract: We compare the Heston model with $ρ=-1$ to the one-dimensional local-volatility model calibrated to the same European option prices. We show that, for each fixed expiry $T>0$, their integrated variances satisfy [ I_T^{\mathrm H}\prec_{\mathrm{cx}} I_T^{\mathrm{LV}}. ] This strict ordering gives a Heston-model counterexample to the convex-order inequality conjectured by J.\ Gatheral