Motion sickness: an evolutionary and genetic basis for the negative reinforcement model

It has been theorized that motion sickness evolved as a negative reinforcement system which terminates motion involving postural instability and/or sensory conflict. A hypothetical example is provided by a "thought experiment" whereby protohominids are in a tree looking for food. Selection pressure results when the organisms that have an aversion to motion-producing sensory conflict do not venture out too far on the tree limbs and therefore tend to survive. In order to support an evolutionary model for motion sickness there must be evidence for genetic and/or heritable predisposition. The present study involves a retrospective literature review which reveals abundant evidence for genetic/heritable factors in motion sickness. Examples include genetic polymorphism of the alpha-2-adrenergic receptor, which has been shown to increase susceptibility to motion sickness, examination of family trees revealing heritable motion sickness susceptibility, evidence indicating that Asians are hyper-susceptible to motion sickness, and twin studies, just to mention a few. Thus, the theory of heritable negative reinforcement as a basis for motion sickness is supported by extensive evidence in the medical literature. This theory is compared and contrasted with other theories. Further areas for research are suggested.