Democracy of Families and Neutrino Mass Matrix

On the basis of a seesaw-type mass matrix model for quarks and leptons, Mf ≃ mLM F mR, wheremL ∝ mR are universal for f = u, d, ν and e (up-quark-, down-quark-, neutrinoand charged lepton-sectors), and MF is given by MF = K(1 + 3bfX) (1 is a 3 × 3 unit matrix, X is a democratic-type matrix and bf is a complex parameter which depends on f , neutrino mass spectrum and mixings are discussed. The model can provide an explanation why mt ≫ mb, while mu ∼ md by taking bu = −1/3, at which the detarminant of MF becomes zero. At bν = −1/2, the model can provide a large νμ-ντ mixing, sin 2θ23 ≃ 1, with mν1 ≪ mν2 ≃ mν3, which is favorable to the atmospheric and solar neutrino data.