Our experiment demonstrates that the human MNS exhibits a gender difference during the observation of action. In accordance with our previous MEG and H-reflex studies 
, female participants suppressed the mu rhythm to a stronger degree than male participants when observing hand actions. In addition, the mu suppression negatively correlated with SQ whereas it positively correlated with the personal distress subscale of the IRI. The EEG mu rhythm can be a potential biomarker of empathic mimicry. Moreover, the gender differences in the human MNS, as noted by differential mu suppressions through EEG analysis here, provide some indirect support to the extreme male brain theory and may also offer some insight to the mirror neuron account in the autism spectrum disorders (ASD)
The gender difference in the mu suppression in the human MNS during action observation may result from nonspecifically physiologic as well as empathic gender differences. Our study, however, controlled several physiologic factors to a certain degree. The female and male participants were of similar age, handedness, and educational level. Neither the guess of the displayed hand sex, the continuous performance task, the mu rhythm during the Baseline, the mu rhythm during the action Execution, the hand muscle EMG change across the observational conditions, nor the perceived sex differences (same sex vs. opposite sex) differed significantly between the genders.
The issue of gender differences in empathy is quite controversial. Indeed, evidence for gender differences in empathy are huge for self-report questionnaires of empathy in which it is obvious what was being indexed, but are smaller or nonexistent for other types of indexes that are less self-evident with regard to their purpose 
. Moreover, adults' self-reports of empathy have been associated with indexes of social desirability in some studies 
. It is therefore crucial to investigate the neurophysiological mechanism that underpins empathy in relation with gender. One crucial aspect of empathy relies on the unconscious emotional mimicry that leads to affective sharing between self and other [23–24; 33]. This sharing stems from the perception–action coupling (supported by the MNS), which automatically induces the observer to resonate with the emotional state of another individual, with the observer emulating the motor representations and associated automatic and somatic responses that stem from the observed target 
. Furthermore, it has been acknowledged that females show superiority in empathy 
and appear to perform better at reading others' facial and body actions while communicating, and score higher on tests of emotional recognition 
. Therefore, the gender differences of the MNS noted here, depicting stronger mu suppressions to the observed hand action in female than in male participants, might arise from gender differences in empathy.
Furthermore, the finding of gender differences in the mu suppression fits well with a growing body of brain imaging and neurophysiological studies. For example, females displayed stronger activation in inferior frontal cortex during emotional speech perception than males 
. Another study indicated that females showed widespread frontal latency reductions of steady-state visual evoked potentials, predominantly right side, associated with the processing of unpleasant images whereas males did not 
. MEG measurements demonstrated that females produced stronger activation than males, in the primary motor cortex when viewing hand action relative to a moving dot 
. Spinal elicited excitability when observing bipedal step is stronger in females than in males 
. Females awakened stronger activities of facial currogator (frowning) and zygomatic (smiling) muscles when viewing angry and happy faces, respectively, than males 
. The EEG mu suppressions support the existence of gender differences as a direct measure of the human MNS activity.
Interestingly, Williams and his colleagues (2001) speculated that consequent developmental failures of the MNS could lead to impaired self-other representations and imitation 
. This, in turn, could lead to impaired social and communication abilities, such as empathy and language, as it is the case in ASD. Recent studies have demonstrated that patients with ASD have abnormal function of the MNS 
. Particularly, a study using EEG mu rhythm reported little mu suppression in individuals with ASD when they observed hand movements 
. Here, similarly, healthy male participants exhibited less mu suppression when visually presented with hand actions than female participants. Further, a negative correlation between mu suppression and SQ was found. Considering that the extreme male brain theory of autism posits that ASD represents an extreme of the male brain pattern with impaired EQ and enhanced SQ 
, the present findings cast some light on the normal male MNS pattern, as measured with EEG, and lend support to the hypothesis of a dysfunctional MNS in ASD.
It is worth noting that the mu suppression during the observation of hand actions positively correlates with the personal distress subscale of the IRI, and negatively correlated with the SQ. A previous study demonstrated that the mu rhythms is sensitive to cognitive and affective influences as well as echo sensorimotor processing in the frontoparietal networks 
. It was suggested that the mu suppression reflects downstream modulation of sensorimotor cortex derived from prefrontal mirror neurons 
. The IRI is probably the most widely used self-report measure of dispositional empathy. Its subscale of personal distress assesses the affective reactions in response to the extreme distress in others. One functional MRI study showed that activity in the human MNS correlates with the score in the perspective taking subscale of the IRI 
. Similarly, one recent MEG study found a significant correlation between the mu rhythm during empathy of pain and the IRI perspective taking subscale 
. The SQ is more difficult to interpret. It supposedly taps the individual drive to analyze or to construct systems. The model of psychological gender differences by Baron-Cohen suggests that there is a major dimension in which the sexes differ, with males being more drawn to systemize than females 
. Here we found that individuals who scored higher in affective response to others and lower in systemizing ability suppress the mu rhythm to a stronger degree when watching hand actions.