The facial width-to-height ratio (fWHR) is a measure of the cheekbone width divided by the height of the face from the upper lip to the brows. The metric is hypothesised to have evolved as an intra-sexual competition mechanism in males, where large fWHRs are thought to signal both threat and aggression. The fWHR is suggested to subtly resemble angry facial expressions, which, in turn, also signal threat. The late positive potential (LPP) and the vertex positive potential (VPP) are two event-related potentials (ERPs) especially sensitive to emotional content. Studies have also found that viewing angry compared to neutral facial expressions elicit a stronger response on the LPP. However, no study has tested how responses to the fWHR and angry facial expressions elicit changes in the LPP or VPP. In this study, participants firstly rated how threatening faces were with either low or high fWHRs with neutral or angry facial expressions. Secondly, EEG-activity was recorded during a picture-viewing task of the same faces. In the first task, participants rated the faces with angry facial expressions as more threatening compared to all other faces, regardless of fWHR, although the high fWHRs were rated as more threatening than the low fWHRs. In the second task, LPP and VPP mean amplitudes were significantly higher for the angry, high fWHR face compared to all other faces tested. This suggests that an additive effect of both angry facial expressions and high fWHRs together creates the highest threat level in both subjective ratings as well as in ERP mean amplitudes. Further ERP research is needed on the relationship between fWHRs and anger to establish how the two features work both separately and together.

The late positive potential (LPP) is an event-related potential (ERP) component associated with increased affective processing. Studies have shown that stimuli with high evolutionary significance (e.g. a threatening face) induce increased activity over centro-parietal areas of the brain. In an electrophysiological context, this is hypothesized to be indexed by greater LPP amplitudes. The facial width-to-height ratio (fWHR) is a facial-masculinity metric which refers to cheekbone width, divided by upper facial height (top of the lip to between the brows). For the first time, LPP amplitudes were examined in subjects upon observing faces with high vs. low facial fWHRs. Prior studies suggest that faces with high fWHRs are perceived as more threatening than faces with low fWHRs. Consequently, fWHR has by some researchers been proposed to serve as a cue of threat. Two separate tasks in the present study were used to investigate this. In the aggression task, males with high fWHRs were judged as more aggressive. Moreover, when put in a threatening context, high fWHR faces also elicited greater LPP amplitudes in subjects compared to faces with low fWHRs. Conversely, in the self-regulation task, differences in LPP amplitudes did not reach significance. In this task, statistical power was low due to few blocks/trials in the ERP experiment and subjects were not primed on threat, which may explain the non-significant results. Taken together, the results provide modest support to the theory that fWHR serve as a cue of threat. Future studies will need to take the present study’s limitations into consideration

In economic games the facial attributes of counterparts bias decisions to trust and decisions to enter play. We report research supporting hypotheses that trust and reciprocation decisions in trust games are biased by mechanisms of sexual selection. Hypotheses that trust game behaviour is modulated by inter-sexual competition were supported. 1) Attractive individuals elicit more cooperation. 2) Male participants display trust and reciprocation toward attractive female counterparts in excess of perceived trustworthiness (and this display is modulated by male self-reported physical dominance). 3) Female participants appear to respond to male trust as a signal of sexual interest and are therefore more likely to exploit the trust of attractive males. 4) In explicitly dating contexts females are more likely to prefer attractive males to pay for the meal. These results indicate that participants are biased by mate choice and mating display considerations while playing economic games in the lab. Hypotheses that trust game behaviour is modulated by intra-sexual competition for resources were also somewhat supported. 1) Male participants reporting an ability to win fights with same-sex peers are more exploitative of other males. 2) Cues to current circulating testosterone level in counterpart’s faces are less trusted but elicit more reciprocation. 3) The male sexually dimorphic trait facial width-to-height ratio (a trait which is related to both aggression and dominance) is related to an increased proportion of decisions to exploit others in the trust game while also being used by others as a cue to untrustworthiness. We conclude that trusting and trustworthy behaviour in both sexes is biased by mating market considerations predicted by intra- and inter-sexual selection.

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