This paper provides a diachronic construction-based explanation of the differential perfective marking conditioned by transitivity status in Western Mande languages, using the Greater Manding group as an exemplar case. This typologically unusual phenomenon has previously been erroneously cast in terms of case alignment, either synchronically (in terms of bidirectional case markers) or historically (in terms of an earlier split-ergative stage). The central insight of my explanation is that the Positive Perfective constructions of the Western Mande languages are multiple-source constructions. The in-depth reconstruction of these constructions presented in the paper provides a theoretically significant illustration of a pattern of repeated emergence of constructional competition in a particular semantic domain, which is subsequently resolved through constructional specialization and merger, resulting in multiple-source constructions and a typologically unusual pattern of differential TAM and polarity marking.