An experiment testing adult Polish speakers’ ability to supply dative forms of unfamiliar nouns revealed strong effects of type frequency (performance was better on inflections that apply to large classes) and neighbourhood density (participants were more likely to supply the target inflection with nonce nouns belonging to densely populated neighbourhoods, i.e., those which are similar to many existing nouns, than with nouns that resemble few words in the language). These findings corroborate two central claims of usage-based theories: that more frequent patterns are more likely to be used productively; and that speakers prefer low-level generalizations over clusters of phonologically similar forms, or clusters of words sharing the same derivational affix, over more global generalizations. The experiment also revealed considerable differences in individual speakers’ ability to inflect nonce words, similar in magnitude to differences in vocabulary size. Performance on the nonce word inflection task was significantly correlated with vocabulary and education. Two further experiments show that the less educated participants were able to supply the appropriate dative forms of real nouns in the same grammatical contexts, and that their difficulties with the nonce word inflection task are not attributable to inability to infer the gender of the nonce word.