Data on the photon nonproportional response of 33 inorganic scintillation materials are systemized and analyzed. The main trends of nonproportionality for different groups of inorganic scintillators, especially for oxides and halides, are highlighted. The dependence of the shape and degree of photon nonproportional response versus chemical composition, dopant type, index of refraction and other fundamental properties of the materials is studied. Better proportionality appears to be correlated with higher index of refraction of the compound. Another related factor is the width of the valence band in halide compounds. With larger valence band width from fluorides, to chlorides, to bromides, and to iodides, a better proportionality is observed.