Here, we demonstrated that the social bond, associated with empathy 
, affects the yawn contagion in humans in terms of occurrence (), frequency (), and response latency ().
Yawn response latency as a function of the social bond.
Social bond overrode social context and nationality differences in explaining the occurrence of contagion and the variation in the response latency. Indeed, yawning is performed by all members of the human species, immediately recognizable, and occurring in all contexts 
. Thus, it is not surprising that yawn contagion is not seriously affected by context or country of origin.
Gender differences in the empathic abilities have been widely reported, with women showing higher empathy levels than men (e.g., 
). Such differences should reflect in dissimilar yawn contagion levels of the two sexes, not revealed by our results (sex was also excluded from the best model). However, another analytical approach is needed for this purpose. In fact, possible gender divergences can only be revealed by considering yawn contagion of dyads belonging to the same social bond category (strangers to kin) and/or by examining the variation trend in the yawn contagion as a function of the social bond within each sex category.
In agreement with previous works, sensory modality did not affect contagion. In 1942, Moore 
first reported that some blind subjects yawned in response to an audio recording of yawns. Arnott et al. 
found that the sound of a yawn, like the sight of someone yawning, was effective at eliciting an urge to yawn and activated part of the mirror neuron area (the right posterior inferior frontal gyrus; pIFG). Additionally, simply reading about yawning is sufficient to trigger yawns, when no sensory cue is involved 
The lack of an effect of the position of the observer with respect to the trigger (frontal, diagonal, or lateral) on yawn contagion matches with Provine's 
observation that yawn-detection process is not axially specific; yawns in orientations of 90°, 180°, and 270° were as potent or nearly as potent as normal, upright, 0° yawns. Moreover, in patients with unilateral destruction of the visual cortex, Tamietto et al. 
found evidence that emotional contagion occurs also when the triggering stimulus cannot be consciously perceived because of cortical blindness.
Contagion insensitivity to sensory modalities and to visual perspective (relative position) and consciousness clearly indicates that the stimulus quality does not play a primary role in triggering the yawning response in the observer. Some authors have questioned that attention differences (with observers paying closer attention to familiar subjects rather than to unfamiliar ones) could account for differences in the yawning response 
. However, heightened arousal (degree of physiological responsivity relative to a baseline) is normally detected in response to novelty whereas diminished arousal is observed in response to perceived familiarity, as a part of the habituation process, an evolutionary adaptation to avoid an unbearable overloading of the attentional system 
The importance of social bond in shaping yawn contagion demonstrates that empathy plays a leading role in the modulation of this phenomenon. Not only is contagion greater between familiar individuals, but it also follows an empathic gradient 
, increasing from strangers to kin-related individuals. Such a gradient holds for both the contagion occurrence (presence/absence; ) and the entity of the response to a given trigger (frequency; ). This is the behavioral confirmation of what clinical, psychological, and neurobiological works have been suggesting over the past decade 
Our findings go further in explaining the linkage between empathy and yawn contagion. In fact, the delay in the yawn response is longer when the trigger is less familiar to the observer (). Perceiving other persons yawning activate a complex network of brain regions related to motor imitation, social behavior, and empathy, which also involves both sensorimotor cortices and limbic and para-limbic structures 
. Thus, the neural regions linked to the emotional sphere of positive affect may be over-stimulated in subjects viewing the yawn of someone they care about. Such over-stimulation may ultimately lead to a potentiated yawning response. A recent study 
which investigated the response of smiles in mother-infant dyads supports this “over-stimulation hypothesis”. The results showed increased activation around the orbitofrontal cortex (OFC) in mothers viewing their own infant's smile compared to an unfamiliar infant's smile. The neuronal processing of positive affect can encompass different types of social interactions, from the mother-infant one to kinship, friendship, and romantic relationships 
. In this case, specific neuronal regions involved in positive affect regulation are activated by both viewing familiar and unfamiliar subjects (infants) but the activation magnitude differs, being greater when social attachment is higher and familiarity are involved 
. A neuro-ethological approach, similar to that used for smiles, should be applied to detect whether the neural pathways of yawn contagion differ as a function of the emotional closeness shared by the first yawner and the responder.
In an evolutionary perspective, the ability to replicate others' yawns, demonstrated in monkeys 
is probably “older” than empathy, only found in human apes and only implied in bonobos and chimpanzees 
. Via yawn replication, social animals can synchronize the behavioral and physiological state of a group 
. However, replication becomes contagion when there is some evidence that an emotional transfer, requiring complex cognitive abilities, is involved. Hence, it is not surprising that the demonstration of a direct link between yawn contagion and emotional closeness in humans follows the trend observed in other primates 
. This is in line with the bottom-up perspective proposed by de Waal and Ferrari 
, who claim that a cognitive continuity bridges non-human to human primates. Indeed, emotional contagion represents an instance of truly affective reactions that may be mediated by neural pathways of old evolutionary origin providing a cornerstone for emotion communication and affect sharing 
. Yawn contagion has been proven greater between group members (compared to extra group ones) in Pan troglodytes 
, and subjects sharing good relationships (measured via grooming) in Theropithecus gelada 
. When considered together, these results suggest that the relationship between yawn contagion and empathy may have developed earlier than the last common ancestor between monkeys, human, and non-human apes.