I consider a very simple model for the evolution of the real space and phase-space density profiles of a dark matter halo as it acquires its mass. I show that this model, which assumes slow growth through gradual accretion, predicts structures much more concentrated than those found in self-consistent simulations of halo formation. By implication, major mergers must act to reduce the concentration of dark matter haloes, adding the orbital energy of the infalling satellite to the internal energy of the pre-existing system and causing it to expand at intermediate radii. This effect should be easy to detect in numerical simulations.