This work calls attention to mesoscopic mass transport as a decisive factor in determining electrocatalytic selectivity. Through multi-scale modeling and a detailed inspection of the experimental literature, we quantify the effect of the so-called "desorption-re-adsorptionreaction" mechanism and show its generality over different redox reactions, different catalysts, and different electrode setups. Importantly, our analysis highlights surface roughness as a key descriptor that can capture the effect of catalyst morphology on selectivity across all relevant length scales. This simple picture thus reduces to (one-dimensional) roughness any influence from morphology, regardless of its origin: catalyst loading, particle shape/size, surface corrugation due to e.g. oxidative pretreatment or electro-polishing, and even alloying.