Contrary to our hypothesis, congruence between facial inferences and real-world performance in three distant cultures (Cameroon, Czech Republic, and Turkey) indicates an outgroup advantage. In an ethnically heterogenous and mixed culture (Brazil), congruence was the lowest for the apparently mixed-race targets. To the best of our knowledge, only three sets of earlier studies have examined the accuracy of face-based inferences regarding personality or behavioural outcome in perceivers from different cultures and both ingroup and outgroup targets24,25,27. Interestingly, Na et al.25 have observed a weak outgroup advantage in accuracy. In their study, competence judgments of American politicians’ faces by Korean (vs. American) perceivers and competence judgments of Korean politicians’ faces by American (vs. Korean) perceivers were stronger predictors for respective election results. Young18 had also observed an outgroup advantage: he performed two experiments focused on identifying fake smiles using minimal groups. It thus seems that at least in some areas related to competition and cooperation, there may be an outgroup advantage in inferential accuracy.

Our results contrast with research that demonstrated an ingroup advantage in other areas of face processing. Decades of research on social cognition28 have established that ingroup members are more likely to be individuated (i.e., perceivers are especially attentive to facial features which distinguish ingroup faces from one another), whereas outgroup members are more likely to be processed categorically (perceivers are attentive to facial features common across outgroup faces). As a consequence, people tend to have more detailed impressions of apparent racial ingroup (as opposed to outgroup) faces29. Such individuation should logically result in a more accurate prediction of a target’s dispositions than categorisation.

Did our perceivers individuate the outgroup targets more than the ingroup ones? It has been shown that perceivers tend to be motivated to paying special attention to outgroup faces that may present a potential threat30,31. This effect is further amplified by activation of a self-protection goal32 and may well be exclusive to male faces33. Given that in our study, perceivers were instructed to focus on aggressiveness in male faces, it is thus possible that they were motivated to process outgroup faces more carefully (i.e., to focus on individuation) in order to determine each target’s threat potential. This would amount to a self-protective response. Our results may therefore represent an extension of motivated face processing directed at outgroup targets, investigated in earlier face memory research4, to the domain of inferential accuracy. Asking perceivers to judge aggressiveness may have primed a certain level of threat and guardedness (especially since the targets were male fighters), just as Young’s18 experiments where perceivers were asked to detect fake smiles may have activated mistrust and guardedness (especially toward outgroup targets). Prima facie, one could expect that such priming might lead to an indiscriminately heightened attribution of aggressiveness to an outgroup. Interestingly, our results show no differences in perceived aggressiveness level attributed to any particular group of faces. It is yet to be seen, however, whether our findings could be generalised to contexts other than aggression or physical competition and to female faces and female raters. This remains a task for future research.

Another way to interpret our findings is to frame them as an ingroup disadvantage and to investigate why perceivers may find it more difficult to process ingroup faces. The greater propensity to individuating ingroup faces may have somewhat paradoxically detracted from the accuracy of inferences by introducing additional cognitive processing29, which diluted the immediate impact of valid facial cues on perception. Similar detrimental effects on deliberation have been reported in other domains such as decision making34.

As mentioned above, the initial Czech sample collected in 2013 showed effects inconsistent with other cultures. We have therefore collected two other Czech samples which show patterns consistent with the other datasets. We have as yet no rational explanation for why the initial Czech sample was difficult to reconcile with the rest of the data. One might speculate whether it could have been related to the media attention given to the so-called ‘immigration crisis’ which escalated in Europe in 2015. Nevertheless, robustness of our current findings should be reassessed in the future studies.

An important limitation of the current study was its failure to assess inter-ethnic contact among the respondents. Based on societal demographics, we assumed that Czechs and Turks have relatively little exposure to faces of African appearance, while Cameroonians are the opposite, and Brazilians have approximately equal exposure to faces of European and African appearance. It has, however, been established that attenuation of the CRE can be the result of inter-ethnic contact measured at an individual level35. Future research should thus examine whether contact plays a similar role for accuracy of facial inferences with respect to behavioural outcomes. If accuracy decreases with increased contact (as the cross-cultural differences seem to suggest), this would present an interesting explanatory challenge.

Data regarding the ethnic origin of the individual fighters were not available on UFC website. Moreover, we gathered no data on how raters perceive the depicted faces in terms of their ethnic similarity. Nevertheless, given a generally high contrast between faces of European and African appearance, we believe that the assumption about perceived ethnic similarity is justified.

Because social-motivational factors can significantly affect face processing17, it is possible that perceivers’ own ethnic identity (and their categorisation of target faces into ethnic ingroup and outgroup with subsequent biases triggered by such a categorisation) may have influenced the current findings9. Given that the extent to which perceivers identify with their ingroup is known to moderate face processing14,36, future research should test the influence of perceivers’ ethnic identity on the accuracy of inferences made from ingroup and outgroup faces.

Finally, facial morphology differs between groups of people, such as heterosexuals and homosexuals37,38, and is related to behaviour39. Future research should thus investigate the facial cues our perceivers used to judge aggressiveness and determine which of these cues actually increased the accuracy of perceivers’ inferences. To wit, it is possible that biased processing of faces led to increased accuracy merely accidentally. It is known that, for instance, perceivers focus more attention on the eye area of ingroup target faces than outgroup faces15. If our perceivers were more likely to attend to other areas of outgroup faces than the eye region, and if those areas happened to contain cues that aided accurate inferences (e.g., relative facial width), their accuracy for outgroup faces may have been higher not despite but because of biased outgroup face processing.

Within the still little body of research that examined face processing across different groups of both targets and perceivers and applied some real-world accuracy criterion, reports of an outgroup advantage are beginning to emerge18,25, countering evidence for an outgroup disadvantage in other face-processing domains. Our findings suggest that these recent reports of an outgroup advantage have captured a real and replicable phenomenon that may have as yet escaped the researchers’ attention. Future research should try to uncover the mechanisms and moderators of this outgroup advantage in inferential accuracy.