Lande’s (1980b, Evolution34: 292–305) equations for predicting the evolution of sexual size dimorphism (SSD) through frequency-dependent sexual selection, and frequency-independent natural selection, were tested against results obtained from a stochastic genetic simulation model. The SSD evolved faster than predicted, due to temporary increases in the genetic variance brought about by directional selection. Predictions for the magnitude of SSD at equilibrium were very accurate for weak sexual selection. With stronger sexual selection the total response was greater than predicted. Large changes in SSD can occur without significant long-term change in the genetic correlation between the sexes. Our results suggest that genetic correlations constrain both the short-term and long-term evolution of SSD less than predicted by the Lande model.