Recently, geneticists have identified large geographic disparities in the relative proportions of Asian ancestry across different genetic systems (NRY, mitochondrial DNA, autosomes and X chromosomes) in Austronesian-speaking societies of Island Southeast Asia and the Pacific.

Surprisingly, a substantial genetic discontinuity occurs in the middle of a continuous chain of islands that form the southern arc of the Indonesian archipelago, near the geographic center of the Austronesian world.

In the absence of geographic barriers to migration, this genetic boundary and swathe of Austronesian language replacement must have emerged from social behavior. Drawing on decades of comparative ethnological research, we propose a two-stage ethnographic model which accounts for the observed linguistic, genetic and cultural patterns.