The main challenge in the standard model calculation of the mass and width difference in the D0-D̄0 system is to estimate the size of SU (3) breaking effects. We prove that D meson mixing occurs in the standard model only at second order in SU (3) violation. We consider the possibility that phase space effects may be the dominant source of SU (3) breaking. We find that y = ΔΓ/2Γ of the order of one percent is natural in the standard model, potentially reducing the sensitivity to new physics of measurements of D meson mixing.