The Principle of Inverse Effectiveness (PoIE) was born 25 years ago next week (22/07/1983). In its youth, PoIE has been immensely influential in multisensory neuroscience, making regular IMRF appearances. PoIE describes an inverse relationship between multisensory integration and the maximal unisensory response. The PoIE is ‘seemingly inviolable’ (Alvarado et al., 2007): It is observed in most multisensory studies of the superior colliculus, auditory cortex, ventral intraparietal area, and in certain aspects of behaviour. Given this omnipresence, what is the alternative or null hypothesis? I describe a Matlab-based statistical search for the null hypothesis, asking: Under which numerical conditions is the PoIE *not* followed? I focus on three candidates: 1) Multisensory responses are uncorrelated with unisensory responses, 2) reflect their sum, or 3) their mean. The PoIE is almost always observed when unisensory and multisensory responses are uncorrelated, but the particular equation used to calculate multisensory integration can reduce the occurrence of PoIE, under uncorrelated conditions, from ubiquity to chance. The inviolability of the PoIE may therefore be due, in large part, to the particular equations used to calculate multisensory integration. The degree to which PoIE reflects the underlying neural processes of multisensory integration is yet to be determined.