In spite of extensive efforts of the spectral and lightcurve observations, the emission mechanism of γ-ray bursts (GRBs) is still unclear. Recently the observational techniques of X-ray and γ-ray polarizations have been developed rapidly, and several satellites with high sensitivity for such high energy polarizations have been planned. They will provide us the first definitive detections of GRB polarizations and enable us to discuss their statistical properties. We calculate the distribution of GRB polarizations in the synchrotron models (with globally ordered magnetic field and with small-scale random magnetic field) and Compton drag model. The results show that the distribution of the polarization degrees of ∼100 bursts detected by a realistic polarimeter satellite, such as POET, may distinguish the three models, and provide direct diagnostics of the emission mechanism, the magnetic field configuration, and the geometry of GRB outflows.