This study considers the criteria of genitive alternation in English, i.e. the alternation between s- and of- genitive cases, in a unified approach under a single criterion. The factors that are believed to influence the choice between the two types of genitives include animacy, topicality, syntactic weight, a final sibilant in the possessor, etc. Those factors interact with each other complicatedly in the choice between the two types of genitive constructions, and some overlaps among them exist implicitly. This paper suggests that the overlapped factor is the ‘efficiency’ in the speaker’s information delivery in language use. We can get an economic result if the multiple allegedly working criteria for the choice between the genitive case constructions are replaced by the single factor ‘efficiency.’