Rare Muon Decays, Natural Lepton Models, and Doubly Charged Leptons

Simple, natural model of leptons are presented in which the rare decays $\mu \to e\gamma$ and $\mu \to 3e$ occur. The models require doubly charged leptons. The experimental lower limit on the mass of doubly charged leptons determines an upper limit on $\frac{\Gamma (\mu \to 3e)}{\Gamma (\mu \to e\gamma )}$. Parity nonconservation in atoms may be small.