The development of the octavolateralis system in fish ancestors created the phenomenon of sensory reafference associated with the fish's own locomotion. Particularly in fish species living and moving in groups, there is a potential to produce complex pressure waves and other water movements interfering with the octavolateralis perception of critical environmental signals. The hypothesis presented is that the development of the octavolateralis system may have initiated, or been a factor in, the evolutionary development of synchronized group locomotion, eventually leading to schooling behaviour. Theoretical models suggest that schooling may be related to a reduction in masking of environmental signals, as well as to survival mechanisms, e. g. confusion of the lateral line and electro-sensory systems of predators by overlapping pressure waves and overlapping electrical fields. The combined effects of reduced masking and predator confusion may help explain why schooling became an evolutionary success. Including pressure waves and other water movements in the model of join, stay or leave decisions might shed some light on fish shoal assortment. A model encompassing the complex effects of synchronized group locomotion on octavolateralis and electro-sensory perception of both prey and predator fish might increase the understanding of schooling behaviour.