In the natural (mask less) condition, positive emotions (happiness, neutrality, surprise) were recognized more accurately than negative emotions such as fear, sadness or disgust. Masking heavily affected emotion comprehension with a 31% decay in recognizability scores (namely, going from 2.31 in the natural condition to 1.59 in the masked condition, on a scale where 0 indicated the “lack of recognition” and 3 stood for “extremely recognizable”). Overall, these findings fit with previous recent literature showing how facemasks reduce emotion recognition accuracy [3, 4, 5, 6, 7, 8]. In our study, face masking was most detrimental for sadness and especially disgust detection, than positive emotions such as happiness. This pattern of results agrees with previous studies, for example Marini et al. [4], finding that sadness was the most affected and happiness the least affected expression by face masking.
However, we found that mask covering did not affect the recognition of angry faces, which replicates some findings obtained with non-digital masks by Noyes and coauthors [2] (see Fig. 7 of their paper), who also found that the mask and sunglasses conditions did not significantly differ in the angry expressions. The primacy of anger among the biologically relevant emotions has been shown by several studies (e.g., [20]).
Conversely, the emotional display whose recognition was most affected by mask covering was disgust (also in Noyes et al.’s [2] study). Indeed, disgust’s more evident markers (nasiolabial lifting and grimacing and/or nose wrinkling) were hidden by surgical masks in the masking condition. At this regard it is known that successful recognition of anger versus disgust requires one to process information located in the eye/brow region (which was disclosed) as opposed to the mouth/nose region (which was covered by masks), respectively [21]. Again, in a study by Ponari et al. [22] where emotion recognition was hampered by stimuli in which an upper or lower half-face showing an emotional expression was combined with a neutral half-face it was shown that neutral lower half-face interfered with recognition of disgust, whereas the neutral upper half (i.e., the eyes area) impaired the recognition of anger. This difference may probably explain the supremacy of anger and the poor recognition of disgust in the present study.
Women better at recognizing sadness and surprise. In our study, females outperformed males in the recognition of sadness and surprise. Several evidences in the literature consistently reported a similar pattern of results for both sadness [23, 24, 25] and surprise [23]. In addition, according to some investigations, women seem to be more sensitive to sadness whereas men seem to be more sensitive to anger [26, 25, 27].
In another study by Montagne and coworkers [23] women were reported to be significantly more accurate than men at identifying sadness and surprise. Furthermore, Li et al. [24]’s study, performed in 1063 participants varying in sex and age, reported that women performed significantly better at recognizing facial expressions of sadness and disgust.
As for the specific effect of masking, Grundmann and coauthors [6] tested 191 participants (52.9% female) and aging form 19 to 79 years and found that emotion-recognition accuracy declined for masked (vs. unmasked) faces. More interestingly, they showed lower accuracy to being male vs. female, being old (vs. young), and to seeing an old (vs. young) target face. In a study by Calbi et al. [3] involving only three affective displays (neutrality, happiness and anger) it was found that female participants gave more negative ratings than male ones when evaluating angry and neutral facial expressions, and more positive ratings when evaluating happy facial expressions. This was discussed in terms of women’ stronger sensibility to face expressivity and better decoding of emotions through facial expressions [28, 29, 30, 31]. Consistently, Hoffmann and coauthors [32] found that women were better at identifying subtle, less intense emotions (such as sadness), but equally good at identifying clearly expressed emotions (such as fear). Apart from that, it is generally believed that women are more sensitive to emotional facial cues [31].
Men better at detecting fear. In this study, males outperformed women in recognizing fearful expressions (especially masked ones). The increased male ability to recognize fear (relying mostly on the processing of the eyes area, with the typical sclera enlargement) when faces were covered by surgical masks, might depend on the fact the eyes were even more focally attended in the masked condition, being the only uncovered face area. However, Sullivan et al. [33], investigating the percentage of time young women and men spent fixating the eyes and mouth areas of facial expressions (including fear), found that both sexes spent 63.6% of their time looking at the eyes (and 36.4% of the time at the mouth) with no difference across sexes.
In the literature, a male advantage in the processing of fearful expressions is not commonly found, except for an fMRI study, observing regional brain responses to face versus shape identification, in which men showed more significant modulations by both fear and anger affective traits than women [24].
On a different verge, Riva et al. [34] have instead found that the observers' ability to detect pain in a female face was lower than their ability to detect pain in male faces, i.e., that male pain faces are more easily processed at the reflexive level. Relatedly, Simon et al. [35] in an fMRI study found that observing male (vs. female) individuals expressing pain activated in the observers a much greater threat-related response, including the activation of the ventromedial prefrontal cortex, posterior and anterior insula, somatosensory areas, and amygdala. In another study, where healthy subjects were provoked by money taken by an opponent and given the opportunity to retaliate, men showed a higher amygdala activation during provocation, and the amygdala activation correlated with trait anger scores in men, but not in women [36]. As well-known amygdala nuclei are the brain structures most involved in fear and threat processing [37].
Summary. Overall, while face masking reduced the comprehension of all facial expressions but anger (conveying an aggressive display), it was most detrimental for sadness and especially disgust detection (conveying a second person, more passive negative state). The larger impairment for the recognition of the above expressions might depend on their mainly relying on the expressivity of mouth (especially sadness: [16, 17]) and nose areas (especially disgust: [2, 21]), which were covered by masks. Instead, the angry expression was totally unaffected by face masking.
In general, women showed a better performance for positive emotions, both in masked and natural conditions, and men for fear recognition (in natural but especially masked conditions). At this regard, it might be interesting to consider that sex differences in the hemispheric activation for emotion processing have been found. Cahill et al. [38] found that enhanced memory for emotional video clips was associated with activity of the right amygdala in men, and of the left amygdala in women. In addition, an fMRI study investigating the emotional response to odors by Royet et al. [39] found a sex difference in the activation of the left orbitofrontal cortex, which was greater in women compared to men. On the other side, Bourne and Watling [40] found that for males, but not females, greater reported use of negative emotion strategies was associated with stronger right hemisphere lateralisation for processing negative emotions. In the light of the well know right/left asymmetry for negative/positive emotions [41, 42] these studies might provide the neural underpinnings for the higher male accuracy in fear recognition (right amygdala), and of the higher female accuracy for detecting subtle positive emotional cues (e.g., [3]), but further investigations are certainly needed to reach a definitive conclusion.
More in general, our study suggests the opportunity of studying the effect of face masking with really worn facemasks (instead of digitally applied ones) because there might be a difference in the way masks elastically respond to underneath facial muscles contractions, by deforming and stretching differently as a function of the facial expression. Furthermore, the typical inhalation associated, for example, to the surprised or fearful reaction (startle response), which results in mask sucking, will not be observable with digitally applied masks.