This paper presents an unconventional argument based on population growth to bolster marginal productivity theory. There is an economy with a single output produced from a number of differ:nt types of labor. Each type of labor is reproduced from that type itself and from the amount of the output devoted to it under some income distributional norm. Any norm which fails to induce convergence to maximal balanced growth is growth dominated, in that the population and income it induces can be overwhelmed eventually. On the maximal balanced growth path, the norm divides output according to marginal productivity.