The body weight of 21st generation mice selected divergently according to their aggression was compared in different population densities. The mice under isolated housing demonstrated higher levels of aggression in both lines. The growth of the mice was similar among the groups of different population densities in both lines. Only the males of the high aggression line showed a large phenotypic variance in the medium and high population densities, whereas no differences were observed among the population densities in males of the low aggression line or in the females of both lines. Our study suggested that the aggression of males was a source of extra variance in the high population density, resulting in heterogeneity of variance and genotype by sex interaction.