Geniole, Shawn; Department of Psychology
      As do many species, humans visually assess the ability and propensity of others to cause trouble or harm (threat potential), although the mechanisms that guide this ability are unknown. One potential mechanism that may underlie advertisements and assessments of threat is the facial width-to-height ratio (face ratio). The overarching goal of this thesis was to test both the ecological validity of the face ratio (i.e., the extent to which it maps onto an individual’s actual threat potential), and its utility in influencing observers’ first impressions of traits related to threat potential. In Chapter 2, I found that men (n = 146) but not women (n = 76) with larger face ratios were more likely to cheat in a lottery for a cash prize than were men with smaller face ratios. In Chapter 3, to better identify the precise social function of the metric, I examined its differential association with two types of threat-related judgements, untrustworthiness and aggressiveness. The face ratio (n of faces = 141) was more strongly linked to observers’ (n = 129) judgements of aggression than to their judgements of trust, although it is possible that this metric advertises threat potential more generally, of which aggression is a best indicator. In Chapter 4 (which extended some preliminary, additional findings from Chapter 3), I found that observers’ (n = 56) judgements of aggression were strongly correlated with the face ratio (n of faces = 25) even when men were bearded, suggesting that this metric could have been operational in our ancestral past when interactions likely involved bearded men. In Chapter 5, I combined effect sizes from experiments conducted from several independent labs and identified significant (albeit weak) associations between the face ratio and actual threat behaviour, and significant (and stronger) associations between the face ratio and judgements of threat potential. Together, this body of work provides initial evidence that the face ratio, and sensitivity to it, may be part of an evolved system designed for advertising and assessing threat in humans, akin to threat assessment systems identified in other species.