The invention relates to a process for preparing an aqueous polymer dispersion comprising reacting, in the presence of a polymeric cationic co-stabilizer (A), one or more monomers (m), and a polymer (B) of a monomer mixture comprising at least one monomer (x) having a primary, secondary or tertiary amine or amide group and at least one olefinic unsaturation, and, at least one cationic monomer (y) having at least one olefinic unsaturation. The invention also relates to a polymer dispersion, a process for preparing a polymeric stabilizer (C), a polymeric stabilizer (C) and a process for producing paper.