Non-Anomalous Discrete R-symmetry Decrees Three Generations
  We show that more than two generations of quarks and leptons are required to have an anomaly free discrete R-symmetry larger than R-parity, provided that the supersymmetric Standard Model can be minimally embedded into a grand unified theory. This connects an explanation for the number of generations with seemingly unrelated problems like supersymmetry breaking, proton decay, the mu problem, and the cosmological constant through a discrete R-symmetry. We also show that three generations is uniquely required by a non-anomalous discrete R-symmetry in classes of grand unified theories such as the ones based on (semi-)simple gauge groups. 
Jason L. Evans, Masahiro Ibe, John Kehayias, and Tsutomu T. Yanagida
hep-ph hep-th