A scheme is proposed to explain the hierarchy of fermion masses. In the model presented, which has no horizontal symmetry, generations pick up masses one by one through radiative corrections as we go to higher orders in perturbation theory. An extra generation of isosinglet heavy fermions play an important role in implementing the scheme.

• Received 14 December 1987

DOI:https://doi.org/10.1103/PhysRevLett.60.1602