关键词: 哈欠, 演化, 交流功能, 心理, 模仿

Abstract:

Seeing someone yawning or hearing the sound of yawn, we may also yawn involuntarily. This phenomenon is labeled as contagious yawning. Contagious yawning is different from spontaneous yawning and might be related to high-level psychological components. In the current review, we try to explore the ultimate and proximate explanations of contagious yawning to better understand this phenomenon.

Phylogenetically, contagious yawning was first observed in primates such as chimpanzees and bonobos besides humans. Then it was also found in sheep, wolfs, dogs, and elephants. However, except for primates and dogs, contagious yawning in other animals was mostly examined in a single study, which needs replications. In the repeatedly confirmed species, contagious yawning showed similar characteristics as in humans. For example, infant chimpanzees were not susceptible to contagious yawning. Compared with the yawn of the unfamiliar conspecifics, the yawn of familiar ones was more contagious. The comparability between human and non-human species suggested that contagious yawning might evolved to serve adaptative functions. Studies have found that baboons yawned to make their canine teeth prominent when encountering predators or in competitive contexts. Accordingly, when canine teeth are broken, they yawn less frequently. It seems that contagious yawning serves communicative functions. Indeed, when danger is detected, individuals could yawn to transmit the signals to let others keep alert. Accordingly, contagious yawning would help synchronize group movement and keep group members alert so to better defend potential threats.

On the proximate level, contagious yawning is influenced by many factors. For the neural aspect, mirror neurons might serve as important neural substates of contagious yawning. Studies found that when people watched yawning videos or heard the sound of yawning, their mirror neurons would activate. Furthermore, the extent of activation was positively related to the frequency of yawning. On the psychological level, contagious yawning might be based on social cognitive ability such as empathy. For example, the age four is not only critical for the emergence of cognitive empathy, but for contagious yawning; children with autism not only have diminished empathy but less contagious yawning. Also, the relation between contagious yawning and empathy gains some direct support - some studies found a positive correlation between them. Moreover, environmental factors also cause an effect on contagious yawning. Compared with summer, outdoor individuals in winter are more susceptible to contagious yawning. Further studies showed that ambient temperature could predict participants’ contagious yawning. Not restricted to temperature, stress also plays a role in contagious yawning. Interestingly, psychological stress and physiological stress have different influences on contagious yawning, with the former facilitating contagious yawning while the latter playing an inhibition role.

Regardless of the above findings, studies on contagious yawning are still rare. More studies are needed to unveil the mechanisms of contagious yawning and meanwhile, some questions should be taken into consideration. First, research methods should be consistent among different studies so as to make them comparable. Besides, studies on contagious yawning were mainly conducted by researchers in neuroscience, which put less weight on psychological variables. But as theories hypothesized, contagious yawning might serve social functions, which implied that social cognitive abilities might be scaffolded on it both phylogenetically and ontogenetically. In future studies, it would help if scholars from different disciplines could study contagious yawning coordinately. After all, it is an inter-discipline phenomenon.

Key words: yawning, evolution, communicative function, psychology, imitation