The influence of impurities on the basal plane stacking fault energy in GaN is investigated using density functional theory. It is found that silicon, indium, magnesium and carbon impurities each reduce the stacking fault energy by introducing changes to the bonding properties of the material. These bonding properties are analysed in terms of Mulliken charges and bond populations. It is found that the reduction in stacking fault energy correlates both with a reduction in the average anion charge and with an increase in the overlap population.