The present invention is a composition comprising a stable aqueous dispersion of polymer particles having a core comprising structural units of: a) a phosphorus acid monomer, b) an acid monomer, c) a multiethylenically unsaturated monomer; and d) a polymerizable ethylenically unsaturated bulk monomer; and a shell comprising structural units of a1) a sulfur acid monomer, and b1) a polymerizable ethylenically unsaturated bulk monomer; wherein the core protuberates from the shell and the ratio of the structural units of the sulfur acid monomer to structural units of carboxylic acid monomer in shell is at least 0.2:1. Control of the proportion of sulfur acid monomer units to carboxylic acid monomer units results in an improvement in the viscosity stability of paint formulations containing latexes possessing acorn morphology without sacrificing hiding.