On the other side, the age shown by a face seems to reduce its attractiveness. For example, a psychoesthetic study (Forte et al., 2015 ) investigated how aging affected perception of attractiveness. In this study, frontal and lateral views of facial photographs of a middle‐aged woman were modified in imaging software to increase apparent face age. Sixty‐six subjects were given a questionnaire and asked to estimate the age of the subject and how “attractive” she appeared. The correlation coefficient between age and attractiveness had a strong inverse relationship in both face views, suggesting that apparent age diminishes face attractiveness. For these reasons we did not expect to find an opposite‐sex preference for aged faces.

In the present study, the possible interaction of faces' sex with their apparent age was also examined, in light of the evolutionary hypothesis according to which the evaluation of facial pleasantness would depend, among other things, on processing of the face for sexual mate evaluation (see, e.g., Law‐Smith, et al., 2006 ; Lu and Chang, 2012 ). In this line of thought, the positive appreciation of a face of the opposite sex would depend on the presumed age of the depicted person, with lower appreciations for unfertile or unhealthy individuals. In this view, judgments of physical attractiveness might be related to judgments of mating or parenting potential (Jack and Schyns, 2015 ). Mating potential would rely on identifying a healthy opposite‐sex individual, communicated by genetically determined, and thus relatively fixed, aspects of the face, such as symmetry, complexion (Perrett et al., 1998 ), and morphology (e.g., a jaw and brow ridge prominence). Consistently, we did not expect to find an opposite‐sex preference for children's faces because of their unsuitability as mates.

In this study, the judges' sex and the possible interaction with faces' sex was considered, in the view of the so‐called opposite‐ vs. same‐sex a dvantage described in face processing literature (Perrett et al., 1998 ; Penton‐Voak et al., 2001 ; Little et al., 2002 ; Cornwell et al., 2004 ; Kranz and Ishai, 2006 ; Rhodes, 2006 ; Proverbio et al., 2010a ), according to which heterosexual individuals would tend to be more attracted (or give higher ratings) to faces of the opposite vs. the same sex. In a previous ERP study (Proverbio et al., 2010), we investigated the neural correlates of this psychological behavior and found that the inattentive perception of opposite‐sex faces (in both genders) was characterized by a larger and earlier N400 electrical response compared with same‐sex faces. These data are consistent with the evolutionary hypothesis that individuals attend more strongly to opposite‐sex faces than to same‐sex faces as a way to facilitate the identification of potential mates.

Several recent neuroimaging studies (Leibenluft et al., 2004 ; Nitschke et al., 2004 ; Strathearn et al., 2008 ; Kringelbach et al., 2008 ; Glocker et al., 2009b ) have investigated the neural circuits subtending the so‐called parental response to infants and identified a set of structures involving predominantly the orbito‐/frontal cortex devoted to social cognition and belonging to the dopaminergic reward system (Galvan et al., 2005 ; Zald et al., 2004 ). Interestingly, some neuroimaging studies (Bartels and Zeki, 2004 ; Leibenluft et al., 2004 ; Nitschke et al., 2004 ) have investigated the neural correlates of maternal love by recording the brain activation of mothers viewing pictures of their own children. The results showed activation of brain areas linked to affect (amygdala) and in particular positive emotions (orbitofrontal cortex and connected regions belonging to the pleasure/reward circuitry such as the periaqueductal gray matter). The possible role of oxytocin in maternal love has been investigated in an electrophysiological study (Peltola et al., 2014 ) testing the associations of motherhood and oxytocin receptor genetic variation with neural and behavioral responses to emotional expressions of infants and adults. It was found that mothers (vs. nonmothers) and individuals carrying the rs53576 GG variant of the OXTR gene (vs. A‐carriers) showed enhanced ERP differentiation of infants' strong‐ vs. mild‐intensity facial expressions (i.e., pleasure and distress vs. comfort and discomfort). Other studies seem to corroborate the hypothesis that women are more responsive to baby schema than men (Alley, 1981 ; Proverbio et al., 2007 ; Glocker et al., 2009a ) and better able to decode infant expressivity (Babchuk et al., 1985 b; Proverbio et al., 2007 ). We previously carried out an electrophysiological study (Proverbio et al., 2006a ) to investigate the neural response to baby schema in female and male adult individuals who were either parents themselves of infants of the same age, or, rather, nulliparous as in the Glocker et al. ( 2009b ) study. ERP results evidenced of a larger sensory P110 response to faces among women than among men (regardless of parental role), a greater P110 response in the left hemisphere in mothers than in nulliparous women, and larger P3 amplitudes in mothers than in all other groups (including fathers and nulliparous women), a finding that may be interpreted as a sign of greater perceptual sensitivity or increased arousal response in mothers at the view of unrelated infants. The left hemispheric involvement of the occipito‐/temporal cortex in women for the processing of human faces has been confirmed in two other independent studies, showing a bilateral pattern of activity of the FFA indexed by N170 ERP response in females, as opposed to the typical male right‐side hemispheric asymmetry (Proverbio et al., 2006b , 2012 ). A similar pattern of results has been shown by Tiedt et al. ( 2013 ). Interhemispheric transfer time of face‐related inputs seems also to be asymmetric across sexes: N170 recorded in men have faster latencies in the left visual field (LVF)/RH→LH (170 msec) direction than in the right visual field (RVF)/LH→RH (185 msec) direction, whereas it is symmetrical in women (Proverbio et al., 2012 ). As for the auditory modality, other studies have demonstrated a female vs. male enhanced response to the infant vocalizations (cry and laugh; Purhonen et al., 2001; Seifritz et al., 2003 ; Sander et al. 2007 ) supporting the hypothesis of a sex difference in the parental response to infantile communicative signals.

In the present study, 15 male and female University students evaluated 400 human faces of various age and sex according to the parameters of arousal and valence. The same face set was preliminarily validated (and sex‐matched) by a group of 20 independent judges (10 men and 10 women) who were asked to evaluate the degree of trust inspired by each face by means of a three‐point Likert scale. The aim was to explore the possible interaction of facial characteristics with judges' gender and age. Participants shared their ethnicity (which was Caucasian) with that of the observed faces (therefore ethnicity and “race” were not factors in this study, nor was the so‐called other‐race effect (ORE; Walker and Tanaka, 2003 ; Caldara et al., 2004 ; Proverbio et al., 2011a ).

Specifically, perception of aversive faces would activate an amygdala‐based arousal response able to affect general stimulus processing (Phelps and LeDoux, 2005 ). Furthermore, stimuli inducing a greater arousal in the percipient would be subjected to a prioritized processing (Harmon‐Jones et al., 2011 ) because of their biological relevance; for example, infant faces would trigger an instinctive parental response (Kringelbach et al., 2008 ; Parson et al., 2013 ). In this respect, it has been also shown that erotic stimuli are particularly arousing for men (compared with women). Sabatinelli and coworkers ( 2004 ) provided evidence that perception of erotic pictures was associated with a much larger activation of the extrastriate visual cortex in men vs. women, whereas Huynh and coworkers ( 2012 ) showed the opposite effect in women, with high‐intensity erotic visual stimuli deactivating the primary visual cortex compared with low‐intensity erotic movies and neutral movies. Conversely, perception of body mutilations (stimulating the empathic circuits) was associated with a stronger activation of the extrastriate visual cortex in women vs. men in Sabatinelli et al.'s fMRI study ( 2004 ). These findings suggest that the degree of cerebral arousal and mobilization of attentional resources devoted to stimulus processing depend on its biological relevance for the observers.

More recently, in a study exploring the role of arousal and valence in a face search task (Lundqvist et al., 2015 ), the authors examined whether differences in rated arousal between angry and happy expressions within a particular stimulus could predict whether the angry or the happy expression would be detected faster when used as targets. They found that the hypothetical anger superiority effect (ASE) or the happy superiority effect (HSE) described in contrasting literature actually reflected differences in arousal (and average expression intensities) rather than an underlying functional evolutionary psychological bias toward a specific emotion. Indeed, arousal is known to influence stimulus processing fundamentally (and specifically face processing; Lundqvist and Öhman, 2005 ; Lundqvist et al., 2015 ), in that it would reflect the degree of arousal and mobilization of attentional resources to detect as efficiently and rapidly as possible face/body signs revealing the direction and consequences of imminent behavior from conspecifics (see, e.g., Russell, 2003 ; Lang and Bradley, 2010 ).

In a neurometabolic study, Lane et al. ( 1999 ) have shown that differences in arousal and valence of visual IAPS (International Affective Picture System; Lang et al., 1988 ) stimuli, a large collection of color photographs, normatively rated for pleasure, arousal, and dominance) were associated with a differential activation of the extrastriate and the anterior temporal cortex (in terms of an enhancement for the more arousing and positive stimuli). Specifically, they scanned six male subjects with PET while they viewed emotional pictures and postscanning evaluated six types of picture sets (e.g., pleasant, unpleasant, neutral, arousing, not arousing, neutral) each consisting of 14 color pictures. Pleasant emotions relative to neutral were associated with activation in the putamen, medial prefrontal cortex, right anterior temporal cortex, and left extrastriate visual cortex. Unpleasant emotions relative to neutral were associated with activation in the right extrastriate visual cortex. Valenced emotions (pleasant and unpleasant emotions relative to neutral) were associated with activation of bilateral extrastriate visual cortex and the right anterior temporal region. It was not possible to look for potential sex differences in the psychological and emotional response to faces because all participants in that study were male.

The aim of the present project was to investigate the role of viewer sex in the emotional evaluation and psychological reactivity to human faces of various age, sex, and typology. According to an influential model (Osgood et al., 1957 ; Russell, 1979), dimensional and emotional reactions to stimuli and events can be characterized by two factors, the valence (pleasant–unpleasant, good–bad, positive–negative) and the intensity of the activation/arousal (the physiological activation or the intensity of the inner emotional reaction: high–low). For this purpose Bradley and Lang ( 1994 ) have developed a nonverbal pictorial assessment technique that directly measures the pleasure, arousal, and dominance associated with a person's affective reaction to a wide variety of stimuli.

The scores attributed by each judge to each of the 400 humans faces were analyzed separately for the arousal and valence dimensions by means of two multifactorial repeated‐measures analysis of variance (ANOVA) whose factors of variability were 1) between group, gender of judge (male or female) and 2) within groups, sex of faces (male or female), and age of faces (children= CHI, adolescents= ADOL, adults= ADU, mature adults= MAT, elderly= ELD). In prior ANOVA testing, the normality of data distribution was measured (by means of kurtosis). Data indeed had a normal distribution, as also confirmed by kurtosis analysis (valence –0.53, arousal 0.36). A post hoc Tukey test was used for comparisons among means.

To assess the two dimensions of valence and arousal, a modified version of the Self Assessment Manikin (SAM; Lang et al., 1997 ), an affective rating system devised by Lang (1980), was used. In this system, a graphic figure depicting values along each of the two dimensions on a continuously varying scale is used to indicate emotional reactions. Figure 2 illustrates the paper‐and‐pencil version of SAM used for face evaluation in the present study. As can be appreciated, for the valence dimension, SAM ranges from a smiling, happy figure to a frowning, unhappy figure. For the arousal dimension, SAM ranges from an excited, wide eyed figure to a relaxed, sleepy figure. Judges could select any of the three figures comprising each scale, which resulted in a 0–2 points rating scale for each dimension. Ratings were scored such that 2 represented a high rating on each dimension (i.e., high arousal, positivity), and 0 represented a low rating on each dimension (i.e., low arousal, negativity), with 1 representing an intermediate score.

The images were randomly ordered in a PowerPoint presentation at one picture per slide and presented to the judges. The experimenter briefly showed them each picture for a few seconds, and then asked them to evaluate its valence and arousal by using a three‐point scale. As the judge gave his or her opinion for each photograph, the experimenter recorded the score. Any potential bias in the responses was minimized by giving each judge a randomly chosen presentation order.

The pictures showed only the subject's face, up to the base of the neck. Their size was 265 × 332 pixels. The pictures may show accessories (e.g., glasses, hats, earrings, etc …) and displayed various emotional expressions, ranging from joy to anger, matched across stimulus categories. Table 2 shows the result of accurate balancing across class ages and facial expressions of persons depicted. Stimuli were equiluminant as ascertained by an ANOVA performed on individual luminance measures obtained via a Minolta luminance meter.

Four hundred color pictures of anonymous human faces were selected from available, open‐access license‐free databases. An example of a stimulus (blurred for privacy's sake) is provided in Figure 1 . Faces were equally represented by sex (200 M, 200 F) and age ranges (children [2–8 years], adolescents [12–18 years], adults [25–35 years], mature adults [35–60 years], and elderly [above 70 years]) as detailed in Table 1 . In the framework of the so‐called contact hypothesis (Rhodes and Anastasi, 2012 ; own‐age bias), for which face familiarity/sensitivity depended on actual frequency of contact, in determining face numerosity per class, it was set a ∼1:2 ratio for ages similar to that of viewers (adults), ∼2:5 for the elderly, and ∼1:5 for children/adolescents.

Participants were 15 right‐handed heterosexual university psychology students with normal or correct vision. They were eight females ranging in age from 21 to 25 years (mean age 22.9, SE 1.40 years) and seven males ranging in age from 21 to 29 years (mean age 25.7, SE = 1.59 years). Half of the female participants used oral contraceptives, and the other half did not. The specific menstrual phase of naturally cycling women was not determined. All participants received academic credits for their participation and provided written consent. The experiment was performed in accordance with relevant guidelines and regulations and was approved by the ethical Committee of the University of Milano‐Bicocca. Participants were unaware of the experimentation's purposes.

The ANOVA performed on valence scores showed a strong effect of gender of subjects (F 1,398 = 7. 41; P < 0.007; eta2 = 0.018). The valence attributed to faces was significantly higher when judges were women (1.2; SE = 0.022) than men (1.1; SE = 0.02), as can be appreciated in Figure 3 B. This result indicates that women had more positive feelings than men in response to faces. The further triple interaction of judge gender × sex of faces × age (F 16,780 = 2.7987; P < 0.0002; eta2 = 0.054) and relative post hoc comparisons showed that, for male participants, the only significant effect of age was found for the category of female faces, which were more positively evaluated for adolescents (1.242; SE = 0.056) than mature women (0.951; SE = 0.056). This effects reflects a strong preference for young faces of the opposite sex, in males. For female participants, the only significant effect of age was found for the category of elderly of either sex that were evaluated more positively by women (F = 1.4, SE = 0.66; M = 1.38 = 0.06) than by men (F = 1.125; SE = 0.66; M = 1.186; SE = 0.06) and more positively by women compared with adolescents, adults, and mature people but equally well to children of either sex (see Fig. 5 for mean values and SEs).

The ANOVA performed on arousal scores showed a very strong effect of gender of subjects (F 1,780 = 46.683; P < 0.000001; eta2 = 0.0056). The self‐perceived arousal induced by face viewing was significantly higher among women (0.643; SE = 0.0198) than among men (0.496; SE = 0.0198), as can be seen in Figure 3 A. This result hints at a sex difference in the intensity of the response to faces. There was a nonsignificant tendency ( P < 0.07) for a gender × sex interaction, driven by female judges evaluating male faces more intensely. The significant interaction of judge gender × sex of faces × age and relative post hoc comparisons (F 16,780 = 6.1; P < 0.000001; eta2 = 0.11) showed a tendency for an opposite sex preference for adolescents and adults (in both sexes) and mature faces (only in women), which did not reach statistical significance, and a strongly significant ( P < 0.0001) effect of gender with an increased arousal in response to children and elderly people (regardless of their sex), as clearly visible in Figure 4 .

DISCUSSION

In this study, the effect of gender of viewers on the way they evaluated 400 human faces in terms of valence (positive vs. negative) and arousal (high vs. low) was investigated. Regardless of faces' sex, women's ratings were significantly higher for both dimensions, suggesting that women might be more interested or aroused by the specific sensory stimulus (the human face), as displayed in Figure 6. These data fit with some electrophysiological literature (see, e.g., Proverbio et al., 2008) providing evidence of a greater female electrocortical responsivity to faces and people than to inanimate scenarios such as landscapes. In Proverbio et al.'s (2008)'s study, an enhanced N2 component of ERPs was found to people, associated with a stronger activation of fusiform face and extrastriate body areas (FFA and EBA) in women. This was interpreted as an index of greater interest in or attention to this class of biologically relevant signals (human faces and bodies). Because, in that study, no behavioral response or attention allocation to persons was required by the task (which consisted of detecting rare Mondrian stimuli), the stronger responsivity to social stimuli in women would reflect a privileged automatic processing of images depicting face of conspecifics in the female brain. Consistent with this hypothesis, numerous studies have demonstrated that women are provided with a greater ability to decipher the emotions through facial expressions (see, e.g., Proverbio et al., 2006, 2007; Hall et al., 2010; Thompson and Voyer, 2014) or nonverbal communication (Hall et al., 1978) and are more inclined to and more competent in expressing their emotional experiences to others (Dimberg and Lundquist, 1990). Further evidence has demonstrated that women, compared with men, react more strongly when viewing affective stimuli (such as IAPS) involving human beings, thus showing a higher empathic response (Klein et al., 2003; Gard and Kring, 2007; Proverbio et al., 2009). In this regard, some authors have established a link among gender, social skills, and action processing because of the strong association between the known action observation/execution properties of the motor mirror system and the theorized social functions of the human mirror system (Oberman et al., 2007). In fact, sex differences have been shown in action understanding tasks. Women have been found to be better at understanding the action purpose, as indexed by earlier and larger discriminative ERP responses to incongruent and purposeless behavior (Proverbio et al., 2010c), compared with men. Consistently, the combined fMRI and ERP study by Canessa et al. (2012) and Proverbio et al. (2011c) found differences across male and female participants involving a stronger activation of the action understanding system, the STS, and the ventral premotor cortex (associated with the mirror resonance of others' actions) during the observation of cooperative (vs. affective) scenes among women. Again, Anderson et al. (2013) provided evidence of sex differences in the development of brain mechanisms for processing biological motion. This fMRI study involving the visual perception of point‐light displays of coherent and scrambled biological motion showed enhanced activity during coherent biological motion perception in females relative to males in a network of brain regions possibly implicated in social perception (including amygdala, medial temporal gyrus, and temporal pole). All in all, these pieces of evidence indicate a female superiority in social skills and sex differences in action/behavior processing.

Figure 6 Open in figure viewer PowerPoint Effect of sex and age of observed faces, resulting from the score analysis of arousal (top) and valence (bottom) attributed by female (left) and male (right) participants.

In light of this framework, the hypothesis can be advanced that the higher female ratings of valence and arousal found in the present study might correspond to a greater attentional allocation or interest for human faces as sensory signals (Pavlova et al., 2014, 2015). In this regard, it is interesting to mention that behavioral and electrophysiological evidence has shown that women are better at seeing faces, even when there are none, a perceptual illusory phenomenon called face pareidolia. In more detail, Pavlova et al. (2015) carried out a spontaneous recognition task in which adult females and males were presented with a set of food‐plate images resembling faces (Arcimboldo style). Women not only more readily recognize the images as faces (they reported images as resembling a face, on which males still did not) but gave overall more face responses. Proverbio and Galli (2016) investigated the neural correlates of this sex difference, in a study in which ERPs were recorded while participants viewed pictures of animals intermixed with those of familiar objects, faces, and faces‐in‐things. Face‐specific vertex positive potential (VPP; 150–190 msec) showed a difference in the processing of faces‐in‐things between males and females at frontal sites; whereas for men VPP was of intermediate amplitude between faces and objects, for women there was no difference in VPP response to faces or faces‐in‐things, suggesting a marked anthropomorphization of objects in the latter group. SwLORETA source reconstruction showed how in the female brain face pareidolia was associated with the activation of brain areas involved in the affective processing of faces (right STS, BA22; posterior cingulate cortex, BA22; orbitofrontal cortex, BA10), which was not found in men.

Among the other results, the present data show that participants tended to attribute a greater valence (reflecting an intrinsic attractiveness of stimuli) to faces of the opposite vs. own sex. However, this effect, known as opposite‐sex preference and predicting that heterosexual adults preferred to view attractive faces of the opposite sex more than attractive faces of the same sex (Fischer et al., 2004a, 2004b; Turk et al., 2004) was not significant per se, except for men in interaction with face age. This finding is fully consistent with the results of the fMRI study by Cloutier and coworkers (2008) reporting an increase in the activation of the nucleus accumbens and the orbitofrontal cortex belonging to the putative reward circuitry, in response to opposite‐sex faces explicitly judged as attractive vs. unattractive, only in male participants, whereas no opposite‐sex preference was found in women. However, another fMRI study (Spreckelmeyer et al., 2013) provided conflicting results showing enhanced activation to cues signaling opposite‐ vs. same‐sex faces in both sexes, along with a similar activation of the brain reward system (including ventral tegmental area, nucleus accumbens, and ventromedial prefrontal cortex).

In our study, the lack of a general opposite‐sex preference might be in part attributed to the interfering effect of face age (because of the presence of individuals not sexually desirable, such as the elderly or little children) and facial expressions. Indeed, half of face stimuli were not happy or smiling but negative or neutral. The aim of the study was not to create a stimulus set of aesthetically attractive faces but (applying an ecological framework) to create a stimulus set of normotypical faces. In this regard, it is worth mentioning that a preference for the opposite sex has not been consistently observed in face processing literature. For example, in the fMRI study by Fischer et al. (2004a), 24 men and women were scanned while they viewed angry, fearful, or neutral faces of either gender. The occipital and anterior cingulate cortices showed an increase in activation during perception of angry male vs. female faces in men, suggesting a more intense male emotional reaction to same‐sex angry faces. In this study, an opposite‐sex bias was not observed, but, in a second study (Fischer et al., 2004b), in which viewers were passively exposed to neutral male and female faces, the authors found that, during exposure to faces of the opposite vs. same sex, men displayed increased activation in the amygdala and in the anterior temporal areas. Therefore, it can be concluded that the perception of emotional faces, and particularly of angry expressions, does not always result in a bias toward opposite‐ vs. same‐sex faces because there is an interaction between face gender and affective valence of the faces.

The present data have general implications with regard to the literature on the opposite‐sex preference, in which face stimuli are usually selected on the basis of high esthetic standards. Indeed when pictures of very attractive males and females, of about the same age as participants, are presented, the “opposite‐ vs. same‐sex” effect is easily found. For example, in Proverbio et al.'s (2010a) study, faces of attractive peers intermixed with target landscapes were presented to heterosexual university students while the EEG was recorded. The results showed that, in both genders, the inattentive perception of faces elicited a larger and earlier N400 response to opposite‐ vs. same‐sex faces whose neural generators lay in face and emotional processing brain areas. In another ERP study (Suyama et al., 2008), P2 responses were found to be larger in response to female faces in men, and male faces in women, during a gender discrimination task.

Other studies based on psychological ratings (Perrett et al., 1998; Cornwell et al., 2004) and brain activation (Kranz and Ishai, 2006) have shown that individuals prefer to view attractive faces of the opposite sex compared with those of the same sex (Penton‐Voak et al., 2001; Little et al., 2002; Rhodes, 2006). This bias may result from the fact that sexually dimorphic facial characteristics convey information about the quality of potential mates. Kranz and Ishai (2006) used fMRI to scan a group of female and male subjects looking at male and female faces. The study took into account both the gender of the faces and the sexual preference of the viewers. Heterosexual women (and homosexual men) exhibited a significantly greater response in the orbitofrontal cortex when viewing male vs. female faces, whereas heterosexual men (and homosexual women) responded more strongly to female than to male faces in an attractiveness rating task. In a similar study, Conway et al. (2008) found that participants preferred a direct (vs. averted) gaze when judging the attractiveness of happy faces relative to unhappy faces and that this preference for direct gaze was particularly pronounced in favor of opposite‐sex faces. Overall, we believe that facial attractiveness (i.e., beauty) is a crucial factor in determining the lack/presence of an opposite‐sex face preference, the other crucial factor being face age. Indeed, another possible reason why the opposite‐ vs. own‐sex preference was not observed as a general pattern in the present study might be related to the presence of many elderly faces (80 of 400 stimuli), whereas the other 110 faces depicted mature adults (ranging in age from 35 to 60 years). Therefore, half of the stimuli faces depicted people much older than the judges, thus not representing optimal choices as sexual mates (see, e.g., Law‐Smith, et al., 2006; Lu and Chang, 2012). Furthermore, faces were (purposely) not selected as particularly pleasant from the esthetic point of view. Faces were normotypical and depicted very average and lay street persons. Indeed, the literature on face‐age effects in face perception has reported a general negative aging stereotype. For example, Ebner (2008) found that old faces were generally judged as less positive than young faces and seen as less attractive and less likeable, but the viewer's sex and its possible interaction with face‐age effects was not investigated in that study.

Notwithstanding the factors previously mentioned, an effect of opposite‐sex preference was found to be statistically significant only in men, in interaction with the age of the faces. Indeed, only for the opposite sex (female), male judges evaluated more positively (in terms of pleasantness/valence) adolescents than mature individuals, although face evaluation did not differ in valence as a function of age for same‐sex (male) faces. This piece of data might be interpreted in light of the evolutionary hypothesis suggesting that heterosexual individuals may appreciate more opposite‐ than same‐sex faces in terms of potential mates (see, e.g., Law‐Smith, et al., 2006; Lu and Chang, 2012), which in turn may explain why the faces of mature women (potentially infertile) suffered a decrease in their valence appreciation. In women, the opposite sex effect was visible as a strong tendency to attribute higher arousal scores to male faces of any age (except for elderly people).

The data allow us to conclude that, although it is not always the case that opposite‐sex faces are judged more positively than own‐sex faces (depending on the affective expression, age, and beauty), the general effect of women evaluating as more positive and more arousing human faces in general (and in particular children and elderly) does not seem to depend on these factors. The female preference for elderly faces might be interpreted in terms of a greater emphatic response (Baron‐Cohen et al., 2004), whereas the female preference for children faces would rely on specific neural mechanisms sensitive to childish cues in face stimuli. Indeed, previous ERP studies (Proverbio et al., 2010b, 2011b) comparing the neural processing of faces of different ages and objects showed a preferential response to children obver adults in the face‐related N170 ERP component in women (and not men), thus indexing a sex difference in the so‐called parental response to baby schema (McKelvie, 1993). It is thought that the visual and the orbitofrontal cortices are specifically activated and aroused by the view of infants, also providing a pleasant sensation through the dopaminergic reward circuitry, and to a greater extent in women than men (Seifritz et al., 2003; Glocker et al., 2009; for a complete review see Hahn and Perret, 2014). Other behavioral studies support the hypothesis that women are more responsive to baby schema than men (Berman et al., 1975; Berman, 1980; Proverbio et al., 2007) and are better able to decode infant expressivity (Babchuk et al., 1985; Proverbio et al., 2007). In an electrophysiological study, Proverbio et al. (2006) investigated the neural response to baby schema in female and male adult individuals, and ERP results revealed a larger sensory P100 response to faces in women than in men (regardless of whether they were parents themselves or nulliparous). These findings may be interpreted as a sign of greater perceptual sensitivity or increased arousal response in women than in men at the view of unrelated infants. Similar studies have shown that infant faces hold greater incentive salience for women than they do for men (Hahn et al., 2013; but see Parsons et al., 2011). Again, infant faces have been shown to capture women's attention to a greater extent that adult faces, whereas infant faces capture men's attention more so than same‐sex faces but much less than opposite‐sex faces (Cárdenas et al., 2013).

In conclusion, the data collected in this study, relative to heterosexual young adults, hint at a sex difference in the evaluation of human faces along the arousal and valence dimensions. Specifically, the preference for people of the opposite sex (with higher valence ratings) was found only in men, in favor of adolescents as opposed to mature women, thus strongly interacting with face age. There was only a tendency for women participants, possibly because of a lack of specific aesthetic value (faces were selected as normotypical), and the presence of negative facial expressions (such as hate, hostility, disgust). A preference for children and elderly faces (with higher arousal ratings) was found only in women, possibly being associated with the greater empathic attitude of the latter. Overall, women (as opposed to men) rated as more arousing and more positive all human faces, possibly indicating a preference for or greater interest in faces, facial expressions, and social information in general (Proverbio et al., 2008), as predicted by the Baron‐Cohen model of sexual dimorphism in empathy and facial expression coding ability (Baron‐Cohen et al., 2001; Baron‐Cohen and Wheelwright, 2004).

One of the limits of this study (the other limit being the relatively small sample) is the lack of control of hormonal cycle or contraception method for female participants. Indeed, it has been shown that social processes, and in particular the neural response to opposite‐sex faces, may vary as a function of hormonal phase of the women.

In addition, Petersen et al. (2014) have shown that oral contraceptive pill use can affect cognition and alter resting‐state functional connectivity. Indeed, women using oral contraceptives have been shown to differ from nonpill users in memory (Mordecai et al., 2008), mental rotation (Griksiene and Ruksenas, 2011), and affective memory tasks (Nielsen et al., 2011, 2014). In conclusion, the hormonal control, or lack of it, represents an important variable in determining the neurofunctional behavior of the female brain, and it should be monitored in studies on sex differences.

Several authors (Alexander and Hines, 2002) have pointed out the genetic/biological nature of female preference for social stimuli. For example, evidence of a toys preference among nonhuman primates (Cercopithecus aethiops sabaeus) has been provided, with male vervets preferring to play with unanimated fast moving toys (e.g., cars or balls) and female vervets preferring the contact with dolls. These data suggest that sexually differentiated interest for infants/dolls arose early in human evolution, prior to the emergence of a distinct hominid lineage. Comparative studies are quite relevant in this regard because monkeys are not subject to the cultural influences proposed to explain human sex differences in social cognition.

Furthermore, other findings support the hypothesis of biological, predetermined sex differences in social interest, not dependent on cultural conditioning but linked to the genetic role of women as primary offspring caregivers. One of the most important pieces of evidence is the observation of an early interest for infants traceable in all human cultures and historical periods in young females. Remarkably, the same phenomenon has been observed in monkeys (juvenile baboons, macaques, and rhesus monkeys; Herman et al., 2003; Maestripieri and Roney, 2006) as reflected by a higher rate of interaction with infants in females than males. The interaction includes behaviors such as embracing, holding, carrying, playing with, grooming, touching, and staying close to infants, and this is unaffected by hormone manipulations. According to Maestripieri and Pelka (2002), sex differences in interest in infants across the life span should be interpreted as a biological adaptation for parenting. Neurohormonal studies carried out in humans have shown that the early interest in infants may be modulated by hormonal factors. For example, Leveroni and Berenbaum (1998) reported that girls precociously exposed to high levels of androgens (because of congenital adrenal hyperplasia) displayed less interest in infants than their normal sisters. Consistently, it has been shown in primates that maternal hormonal changes influence social interaction with unrelated infants (Ramirez et al., 2004), making adult females more empathic and receptive. At this regard, oxytocin has been show to affect the empathic attitude in humans, by increasing social trust and even improving the ability to infer the affective mental states of others (Domes et al., 2007).

On the basis of a review of the relevant literature. many of the sex differences in social cognition (including the present sex difference in face processing) may be related to the (biologically determined) role of females as primary offspring caregivers (as opposed to fighters/hunters; see, e.g., Kuhn and Stiner, 2006). This distinction may be associated with females' greater empathic attitude, ability to understand body language and facial expressions, attachment and responsivity to infants (oxytocin‐mediated), early interest in infants, interest in social information, emotional responsivity, and lesser incidence of autistic, psychopathic, and sociopathic disorders. In this way, this article provides a unified framework for understanding the multifaceted consequences of a sexual dimorphism in human parental behavior.