Charnov & Finerty's (1980) hypothesis of the impact of relatedness between individuals on the multiannual fluctuations of microtine populations (cyclicity) has initiated a number of field experiments in the Holarctic region (Kawata 1987; Boonstra & Hogg 1988, Ylönen, et al. 1990). During recent years, the Charnov-Finerty hypothesis has been shown to be insufficient in explaining population cycles (e.g. Kawata 1990, Pugh & Tamarin 1990, Stenseth & Lomnicki 1990). Lambin & Krebs (1991) presented a new general model for all microtines of the impact of relatedness on the population fluctuations. In the present paper we go through some of the assumptions of the Lambin-Krebs model about the formation of social spring groups, occurence of spring declines and their possible impact on the following summer's demography. The paper is based on our long-term data on social organization and demography of the bank vole Clethrinonomys glareolus but studies on other Clethrionomus species and the field vole Microtus agrestis are also referred to. We attempt to verify the general validity of the model and present some new data on the impact of kinship and familiarity on the population growth in cyclic bank vole (Clethrionomys glareolus) populations.