In the present study we explored for the first time the ability of adult identical twins to recognize their own faces with respect to their co-twin’s face and the face of highly familiar individuals. This ability was also examined in control subjects who were highly familiar with the twins.

The results showed that the control group was better at recognizing their own face with respect to the twins’ faces, which were not discriminated between each other. This result is in line with previous literature indicating that there is something uniquely associated with self-recognition, and that, for control subjects, one’s own face has a particular status compared to other facial identities, regardless of their familiarity. Indeed, the self-face is recognized faster than both familiar and unknown faces [11,12,14–16,61].

Our study demonstrates that monozygotic (identical) twins do not demonstrate this self-face advantage, as their performance was comparable for self-face recognition and for the recognition of co-twin’s face. Tellingly, this (in)ability was predicted by the perceived physical similarity with the co-twin’s face and also by the tendency to have an insecure attachment style.

Self-face recognition in twins

As previously mentioned, self-face has special status for the individual. In general, people have had daily exposure with their own faces for throughout most of their lives and are thus extremely familiar with seeing it. Moreover the self-face has a special salience and more emotional relevance than other faces. It is thus no surprise that personal familiarity, kinship, physical similarity, exposure, as well as the emotional salience of one's own face, influence the facial recognition processes [43–46] and impact the core network of visual areas as well as in extended cognitive and emotional systems [53,54]. Importantly, comparing the self-face with an identical co-twin’s face ensures that all the aforementioned factors are controlled for, and presents an opportunity for researchers to test the unique processes underlying visual self-face recognition. One recent study addressed this issue by investigating the electrophysiological correlates of the self- and dizygotic (non-identical) co-twin’s face [62]. This study showed that self and co-twin’s faces share very similar featural, configural, and matching processes, but differ with respect to the high-order stages of face processing. More specifically when the self-face was compared with a face associated with similar levels of lifetime exposure (and thus highly familiar and emotionally salient), the neural processes of visual self-recognition were similar at early stages indexed by analysis of visual components in 100–300 ms time range, which is sensitive to familiarity and learning effects. In contrast, unique processes seem to characterize later stages (in the 400–700 ms time-range) where the effects of facial-identity has important modulatory effects [63–66]. Also, the fact that self and twin recognition shared very similar electrophysiological signatures supports the ‘self-referent phenotype’ matching theory, i.e. that we recognize our kin by implicitly or explicitly comparing the similarity of other people’s appearance to our own [67]. Moreover, this may suggest that the degree of similarity in the neural processes for recognizing self and kin should also increase when the similarity between self and kin increases [53]. However, the ultimate confirmation of this prediction would come from the comparison of participants who view the self-face and the identical (i.e., monozygotic) twin’s face. Here, we show that, at least at a behavioral level (and with fast stimuli presentation), monozygotic twins fail to discriminate the self-face from the co-twin’s face and that the lack of self-face advantage is predicted by the reported levels physical similarity between the participant and the model. These results are congruent with two studies [17,65] that investigated the effect of physical similarity on self- and familiar- face recognition and found a specific impairment in self-face recognition when the self- and other's face were highly similar. Kircher et al. [22] compared the response times for recognizing the self and highly familiar, overlearned faces obtained by morphing a person’s own face or a friend’s face with a non familiar face. Results showed a significantly slower processing speed when the morphed face contained about 70–80% of the self (and 20–30% of the other face), but not when the self-face was morphed less extensively (with less than 15% of the other face) and thus was less ambiguous as compared to the overlearned face. Yoon and Kircher [68] compared recognition of self, familiar and unknown faces morphed with unknown faces, which were similar or dissimilar to the three identities. Results showed that there were delayed reaction times for the morphed images of "self versus similarly rated unknown faces" compared to the images of "self versus dissimilarly rated unknown faces". Such significant differences were not found for the other morphed versions.

Plausibly, this delayed recognition effect is due to a mismatch between the internal representation of the self-face and the observed self-similar face, artificially obtained with the morphing procedure. Such mismatch becomes relevant only in the case where self-similarity is involved. Indeed, the effect is selective for the self but not for the other's face, even when it is familiar or overlearned, suggesting the identity proof for the self is more complex and requires longer response times. In other words, we posit that, although processing the self has a special status, when the self-face is compared to another self-similar face the performance may be sub-optimal. This may explain why in the case when facial physical similarity becomes maximal (i.e. for monozygotic twins), participants are less able to discriminate the self- from the highly self-similar co-twin’s face to the point that their performance is lower even with respect to discriminating the face of a friend. However, given their high similarity to each other and dissimilarity from others, twins may still maintain a special representation of the self and of the co-twin face as a pair that take to a sort of “self-twin” processing advantage. We tested this hypothesis by comparing the performance of twins and friends (controls) when processing the co-twins’ faces. Twins were better than controls at processing their own and their co-twin upright faces (thus confirming our prediction). This comparison also ruled out the idea that a general difficulty in discriminating two faces with highly similar facial features was the only factor that could explain the lack of distinction between the two faces in both groups.

All in all, based on studies about the neural processing of the self- and dizygotic co-twin face [62], our pattern of results suggest that identical twins may rely upon similar neural processes for visual recognition of themselves and their co-twin, while non-identical twin participants may have to use different resources. Visual self-face recognition entails matching an image of the self to an internal representation of the self. Developmental and cognitive neuroscience studies have recently suggested [69,70] that internal representations of the self-face are built upon accumulating congruent multisensory experiences in which one’s own sensorimotor experience is matched with the face he/she sees in the mirror. This process ultimately allows one to self-identify with the face seen in the mirror [69–71]. The fact that the representation of one’s own face is based on the view that we perceive it in the mirror in everyday life has also been suggested by studies that found faster reaction times for the frontal rather than profile view of the self-face (while no such effect was found for familiar faces, which are usually perceived from many viewpoints) [72]. Also, the continuous updating of matching multisensory stimuli allows one to update the internal self-face representation and thus to maintain the sense of self-identity across gradual changes of one’s own facial appearance due to the passage of time. In monozygotic twins, the stored visual representations of the self and the monozygotic co-twin face, most likely overlap to a very high extent. Thus, we might speculate that self-recognition in twins might rely more heavily on the congruency of multisensory signals than on a stored visual representation of the self-face, which shares too many features with the co-twin’s face.