Effects of 25 generations of positive and negative phenotypic assortative mating were compared with that of random mating in unselected (all individuals reproducing) and randomly chosen (25% of individuals reproducing) populations of various size. The trait under assortation was controlled by either 100 or 2500 loci. Twenty‐five generations of positive assortative mating in large unselected populations (effective population size equal to 1600) caused an accumulating increase in additive genetic variance(VA)causingVAto become 440% of the base population'sVA.Negative assortative mating had a small asymptotic effect in the opposite direction, causingVAto become 72% of the base population'sVA.Changes inVAwere due almost exclusively to gametic phase (linkage) disequilibrium. Positive and negative assortative mating were able to slightly increase or decrease the inbreeding coefficient and the degree of homozygosity. Number of loci, population size and structure had either negligible effects in combination with negative assortative mating or worked in the direction of accentuating the effects of positive assortative mating.