Although motion sickness is not likely to be mendelian, it appears to show familial aggregation. Hence, the hypothesis of Treisman (1977) is of interest. The hypothesis is based on the idea that 'motion sickness is triggered by difficulties which arise in the programming of movements of the eyes or head when the relations between the spatial frameworks...are repeatedly and unpredictably perturbed. Such perturbations may be produced by certain types of motion, or by disturbances in sensory input or motor control produced by ingested toxins.' In nature the last would be important. Emesis in response to motion is viewed by this hypothesis as an unfortunate accidental byproduct of the system to get rid of neurotoxins.