Additive suitable for increasing hardness in the production of flexible polyurethane foam, comprising at least one compound (i) containing at least three hydrogen atoms reactive toward isocyanates, (ii) having an average hydroxyl number, determined to DIN 53240-1:2013-06, of 80-280 mg KOH/g, preferably of 100-250 mg KOH/g, and (iii) containing > 50 wt% of ethylene oxide bound within the molecule, wt% based on the total alkylene oxide content of the compound, wherein the inventive compound has at least 50% primary OH groups and is selected from compounds of the formula in which R 1 = starter substance radical minus the hydrogen atoms active for the alkoxylation, such as preferably molecular residues of polyhydric alcohols, polyfunctional amines, polyhydric thiols, carboxylic acids, amino alcohols, aminocarboxylic acids, thio alcohols, hydroxyl esters, polyether polyols, polyester polyols, polyester ether polyols, polycarbonate polyols, polyethyleneimines, polyether amines, polyether thiols, polyacrylate polyols, castor oil, of mono-, di- or triglycerides of ricinoleic acid, chemically modified mono-, di- and/or triglycerides of fatty acids and/or C1-C24 alkyl fatty acid esters containing an average of at least 3 OH groups per molecule, R 2 is CH 2 -CH(CH 3), R 3 is CH 2 -CH 2, R 4 is CH 2 -CH(R 5), CH(R 6)-CH(R 6), CH 2 -C(R 6) 2, C(R 6) 2 -C(R 6) 2, CH 2 -CH-CH 2 -R 8, C 6 H 6 -CH-CH 2, C 6 H 6 -C(CH 3)-CH 2, molecular residue of mono- or polyepoxidized fats or oils as mono-, di- and triglycerides or molecular residue of mono- or polyepoxidized fatty acids or the C 1 -C 24 -alkyl esters thereof, R 5 is a C 2 to C 24 alkyl radical or alkene radical, which may be linear or branched, R 6 is a C 2 to C 24 alkyl radical or alkene radical, which may be linear or branched, R 7 is a C 3 to C 6 alkyl radical in linear arrangement, R 8 is OH, Cl, OCH 3, OCH 2 -CH 3, O-CH 2 -CH=CH 2, O-CH=CH 2, and where u i ≥ 1, v i ≥ 1, w i is integers of 0-400, especially w i = 0 n is an integer of 3 to 25, preferably of 3 to 10, especially 3 to 8, i is an integer with i = 1 to n, where the sequence of the monomer units in the individual polymer chains 1 to n is arbitrary, and where the compositions of the n-polymer chains may be independent of one another, where the ethylene oxide bound within the molecule is in a terminal position at least to an extent of 90%, % based on the total amount of ethylene oxide bound within the molecule, and where the compounds present contain 55 to 99 wt% of ethylene oxide and 1 to 45 wt% of propylene oxide, wt% based on the total alkylene oxide content of the compound as per formula. (i) containing at least three hydrogen atoms reactive toward isocyanates, (ii) having an average hydroxyl number, determined to DIN 53240-1:2013-06, of 80-280 mg KOH/g, preferably of 100-250 mg KOH/g, and (iii) containing > 50 wt% of ethylene oxide bound within the molecule, wt% based on the total alkylene oxide content of the compound, R 1 = starter substance radical minus the hydrogen atoms active for the alkoxylation, such as preferably molecular residues of polyhydric alcohols, polyfunctional amines, polyhydric thiols, carboxylic acids, amino alcohols, aminocarboxylic acids, thio alcohols, hydroxyl esters, polyether polyols, polyester polyols, polyester ether polyols, polycarbonate polyols, polyethyleneimines, polyether amines, polyether thiols, polyacrylate polyols, castor oil, of mono-, di- or triglycerides of ricinoleic acid, chemically modified mono-, di- and/or triglycerides of fatty acids and/or C1-C24 alkyl fatty acid esters containing an average of at least 3 OH groups per molecule, R 2 is CH 2 -CH(CH 3), R 3 is CH 2 -CH 2, R 4 is CH 2 -CH(R 5), CH(R 6)-CH(R 6), CH 2 -C(R 6) 2, C(R 6) 2 -C(R 6) 2, CH 2 -CH-CH 2 -R 8, C 6 H 6 -CH-CH 2, C 6 H 6 -C(CH 3)-CH 2, molecular residue of mono- or polyepoxidized fats or oils as mono-, di- and triglycerides or molecular residue of mono- or polyepoxidized fatty acids or the C 1 -C 24 -alkyl esters thereof, R 5 is a C 2 to C 24 alkyl radical or alkene radical, which may be linear or branched, R 6 is a C 2 to C 24 alkyl radical or alkene radical, which may be linear or branched, R 7 is a C 3 to C 6 alkyl radical in linear arrangement, R 8 is OH, Cl, OCH 3, OCH 2 -CH 3, O-CH 2 -CH=CH 2, O-CH=CH 2, u i ≥ 1, v i ≥ 1, w i is integers of 0-400, especially w i = 0 n is an integer of 3 to 25, preferably of 3 to 10, especially 3 to 8, i is an integer with i = 1 to n, the sequence of the monomer units in the individual polymer chains 1 to n is arbitrary, and where the compositions of the n-polymer chains may be independent of one another, where the ethylene oxide bound within the molecule is in a terminal position at least to an extent of 90%, % based on the total amount of ethylene oxide bound within the molecule, and where the compounds present contain 55 to 99 wt% of ethylene oxide and 1 to 45 wt% of propylene oxide, wt% based on the total alkylene oxide content of the compound as per formula.