  Based on hybrid density-functional calculations, we propose that
ferromagnetism in the prototypical bixbyite sesquioxide In$_2$O$_3$ doped with
Cr is due to Cr-oxygen vacancy complexes, while isolated Cr cannot support
carrier-mediated magnetic coupling. Our proposal is consistent with
experimental facts such as the onset of ferromagnetism in O-lean conditions
only, the low or vanishing net moment in unintentionally doped material, and
its increase upon intentional doping.
