According to Christopher Bergland for Psychology Today, a universal phenomenon causes approximately half of all adults to yawn after another person yawns. While researchers initially believed this was related to human empathy, one comprehensive study suggests that the true culprits are age and genetics.
Laboratory studies attempting to link the contagious yawning phenomenon to a single cause have been largely unsuccessful, according to Bergland. Popular theories about the factors that affect contagious yawning range from intelligence to empathy levels, but neither have been shown to have a strong link. One study found a solid link between contagious yawning and age, indicating that contagious yawning decreases with age.
While yawning is not directly linked to empathy, a study suggests that a lack of the contagious yawning instinct could be used to diagnose developmental disorders, according to Emily Sohn for Discovery Channel. This finding shows that children with developmental disorders, such as autism, may lack the emotional recognition in others that is necessary for contagious yawning to occur. The behavior is also found only in some humans, dogs and chimpanzees, all species that are linked by high levels of emotional intelligence. While those who do not experience contagious yawning may not actually lack empathy, future studies are expected to explain the emotional component of the phenomenon.