Although Japanese is a nominative-accusative language, the subject of an adnominal clause in the language can also be optionally marked for genitive, which is often called ‘nominative/genitive conversion (NGC).’ However, the observed range of the case conversion, which had been much freer in some 200 years or before, is known to have been shrinking and is more and more limited to an adnominal clause whose predicate is a stative verb or an adjective, in the last 100 years. This chapter claims that such a diachronic change in the possibility of the NGC is due to an interplay between the successive change in the value of the microparameter concerning the syntactic size of the genitive subject clause (GSC) and the Minimal Structure Principle, a kind of economy principle, and demonstrates that the claim is justified by the result of a large-scale experiment of acceptability judgment administered over the subjects between twenties and sixties of age. More specifically, the acceptability of the possessional adjectival sentences, property-describing adjectival (PDA) sentences, and copulative nominal-predicate sentences with a nominative or genitive subject are compared across three different age groups, and only the PDA sentences show an intergenerational variation in acceptability, as predicted.