Relaxor behavior of diffuse phase transition and large frequency dispersion is found among Pb-containing complex perovskites. With slightly different compositions or structures, these same complex perovskites also exhibit normal ferroelectric (FE) or antiferroelectric (AFE) behavior. The AFE transitions involve a volume decrease and are enhanced by increasing the size mismatch between ferroelectrically active B HH cations and ferroelectrically inactive B H cations. This is in contrast to the FE transitions which involve a volume increase and are enhanced by enlarging the space available for ferroelectrically active B HH cations. In relaxors, the size of B H cations is always larger than that of B HH cations, so an increase in the size mismatch is accompanied by a parallel increase in the molar volume. Therefore, a competition between AFE and FE couplings is always present. Such AFE/FE competition has an analogy in magnetism: the competition between antiferromagnets and ferromagnets gives rise to a spin glass behavior in the intermediate random alloys between antiferromagnets and ferromagnets. Evidence in support of this concept is found in the structural characteristics, transformation volumes, pressure dependencies, transition/ depolarization temperatures, and phase diagrams of relaxors ferroelectrics and related compounds.