Several models of regularities behind plural overgeneralization patterns of four-year-old monolingual Germans and bi-/multilingual children acquiring German were empirically examined within the framework of single- and dual-route models. The factors taken into account were frequency, cue validity, productivity, and iconicity of plural allomorphs. In this large-scale cross-sectional study, the results of 893 children tested with a modified, validated version of the speech and language screening MSS and of 476 children tested with a validated language test SETK 3-5 were analyzed. For all overgeneralization models, paired groups of expected and not expected answers were compared in the Wilcoxon test. At the early stages of plural acquisition the frequency of plural allomorphs and the simplest phonetic–phonologically based regularities seem to be the crucial factors recognized by children acquiring German. Gender-based plural allomorph frequency was associated with advanced German skills. These findings support the single-route theory with its emphasis on the influence of input characteristics on children’s error patterns. No evidence was found for a default status of -s or any other plural marker, and hence for the dual-route theory.