Described is an aqueous polymer dispersion comprising a chain growth addition polymer obtainable via free-radically initiated emulsion polymerization from ethylenically unsaturated, free-radically polymerizable monomers, wherein the chain growth addition polymer is formed to an extent of not less than 60 wt % from primary monomers selected from vinylaromatic compounds, conjugated aliphatic dienes, vinyl esters of saturated C1- to C20-carboxylic acids, esters of acrylic acid or of methacrylic acid with monohydric C1- to C20-alcohols, aliphatic hydrocarbons having 2 to 8 carbon atoms and one or two double bonds, or mixtures thereof, wherein the monomers are polymerized in the presence of lignosulfonate and wherein the chain growth addition polymer is branched or crosslinked. Also described is a process for preparing the aqueous polymer dispersion. The polymer dispersion is useful for preparing paper coating compositions.