This is a re-publication, as a citation classic, of a paper published in Genetical Research (precursor of Genetics Research) in 1995. The effective population size is required to predict the rate of inbreeding and loss of genetic variation in wildlife. Since only census population size is normally available, it is critical to know the ratio of effective to actual population size (Ne/N). Published estimates of Ne/N (192 from 102 species) were analysed to identify major variables affecting the ratio, and to obtain a comprehensive estimate of the ratio with all relevant variables included. The five most important variables explaining variation among estimates, in order of importance, were fluctuation in population size, variance in family size, form of N used (adults v. breeders v. total size), taxonomic group and unequal sex-ratio. There were no significant effects on the ratio of high v. low fecundity, demographic v. genetic methods of estimation, or of overlapping v. non-overlapping generations when the same variables were included in estimates. Comprehensive estimates of Ne/N (that included the effects of fluctuation in population size, variance in family size and unequal sex-ratio) averaged only 0.10-0.11. Wildlife populations have much smaller effective population sizes than previously recognized.