The yawn-reflex is seen in many animals, as well as humans, and appears to be an evolutionary conserved behaviour within vertebrates. The functions and mechanisms underlying this behaviour have been under discussion for centuries and a clear answer has not yet been found. This thesis explores the different hypotheses, finding that a lot of physiological hypotheses are lacking in evidence (Ear pressure, blood regulation and state change hypotheses) while others have negative evidence (Oxygen shortage and thermoregulation hypotheses). The arousal and muscle tone relaxation hypotheses also lack evidence but might, with more research, provide interesting answers. It has also been speculated that yawning serves a communicative role. The evidence for contagious yawning is good and appears to be a sign of empathy, but the necessary evidence for the proposed purpose of yawning is lacking in all the social hypotheses. The question of why we yawn has therefore not yet been satisfactorily answered by science.
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