AUTISTIC TRAITS PREDICT TRUST DECISIONS 1
Facial impressions of trustworthiness guide social decisions in the general population, as shown by financial lending in economic Trust Games. As an exception, autistic boys fail to use facial impressions to guide trust decisions, despite forming typical facial trustworthiness impressions (Ewing et al. 2015) Here, we tested whether this dissociation between forming and using facial impressions of trustworthiness extends to neurotypical men with high levels of autistic traits. Forty six Caucasian men completed a multi turn Trust Game, a facial trustworthiness impressions task, the Autism Spectrum Quotient, and two Theory of Mind tasks. As hypothesized, participants' levels of autistic traits had no observed effect on the impressions formed, but negatively predicted the use of those impressions in trust decisions. Thus, the dissociation between forming and using facial impressions of trustworthiness extends to the broader autism phenotype. More broadly, our results identify autistic traits as an important source of individual variation in the use of facial impressions to guide behaviour. Interestingly, failure to use these impressions could potentially represent rational behaviour, given their limited validity. AUTISTIC TRAITS PREDICT TRUST DECISIONS 4 Should I Trust You? Autistic Traits Predict Reduced Appearance Based Trust Decisions Human faces convey rich social information about an individual's perceived personality (Zebrowitz, 2005) For example, a 100ms exposure to a novel face is enough for consistent first impressions of trustworthiness to form (Todorov, Pakrashi, Oosterhof, 2009; Willis Todorov, 2006) Rapid evaluation of trustworthiness from faces is important for survival as it can signal whether someone should be avoided or approached. These evaluations are well understood (Todorov, Olivola, Dotsch, Mende Siedlecki, 2015) and based on underlying cues such as emotional expression, gender, and attractiveness (Oosterhof Todorov, 2008; Sutherland et al. 2013; 2016) Although facial impressions of trustworthiness are formed automatically, they are not especially accurate predictors of trustworthy behaviour (Rule, Krendl, Ivcevic, Ambady, 2013) Regardless of their validity, facial impressions of trustworthiness can have a critical impact on social decisions in everyday life (Olivola, Funk, Todorov, 2014) For example, more trustworthy appearing host profile photos on Airbnb receive more bookings (Ert, Fleischer, Magen, 2016) Surprisingly, host profile photos still influence booking decisions even when more valid information is available in the form of guest reviews. In a financial setting, borrowers on a peer to peer lending site are more likely to be funded if they had trustworthy appearing photographs (Duarte, Siegel, Young, 2012) In economic Trust Games run in controlled laboratory settings, facial impressions of partner trustworthiness also guide trust decisions, even when accurate information about trustworthy or untrustworthy behaviour is also available (e.g. Chang, Doll, van't Wout, Frank, Sanfey, 2010) Together, this evidence suggests that facial impressions of trustworthiness are readily formed from faces and can have powerful effects on trust. AUTISTIC TRAITS PREDICT TRUST DECISIONS 5 Importantly, although people show agreement in their facial impressions of trustworthiness (Rule et al. 2013) emerging research also indicates considerable individual variation in these impressions (Hehman, Sutherland, Flake, Slepian, 2017) Hehman and colleagues (2017) found that variation in observers influenced facial impressions of trustworthiness as much as variation in the faces themselves. Understanding how individuals vary is one of the oldest psychological questions, yet critically, we have little understanding of the factors driving this observer variation in facial impressions of trustworthiness. Even more importantly, we also lack a clear understanding of how and why people differ in the extent to which they use these impressions to guide trust decisions, which is surprising given the critical social consequences of misplaced trust. One key potential source of this observer variation is autistic traits. Specifically, autism (American Psychiatric Association, 2013) may represent one extreme end of a continuous distribution of differences in social and cognitive style which extends to levels of non clinical significance in relatives and the typical population: the 'broader autism phenotype' (Baron Cohen, Wheelwright, Skinner, Martin, Clubley, 2001; Landry Chouinard, 2016; Piven, Palmer, Jacobi, Childress, Arndt, 1997) Given the social communication and interaction differences associated with autism, variation in autistic traits is a good candidate for a source of individual variation in the formation and use of facial impressions of trustworthiness. Indeed, there is strong behavioural and neuroimaging evidence for divergent face processing in autistic individuals as well as neurotypical individuals with high levels of autistic traits (Dawson, Webb, McPartland, 2005; Deruelle, Rondan, Gepner, Tardif, 2004; Halliday, MacDonald, Sherf, Tanaka, 2014; Pierce, Muller, Ambrose, Allen, Courchesne, 2001) For example, compared to their non autistic peers, autistic individuals show reduced ability and/or atypical patterns of neural activation when judging facial AUTISTIC TRAITS PREDICT TRUST DECISIONS 6 emotional expressions (Critchley et al. 2000; Uljarevic Hamilton, 2013) and when inferring social intentions from faces (Baron Cohen, Wheelwright, Hill, Raste, Plumb, 2001) Given that a key facial cue to trustworthiness impressions is emotional expression and that facial impressions involve judging intentions (Oosterhof Todorov, 2008; Sutherland et al. 2016) autistic traits could affect facial impressions of trustworthiness. Consistent with this view, autistic people show atypical neural activation in brain regions associated with facial trustworthiness processing, such as the amygdala (Engell, Haxby, Todorov, 2007) Despite this evidence for atypical face processing, autistic people do not differ from their non autistic peers when explicitly asked to form trustworthy impressions (Ewing, Caulfield, Read, Rhodes, 2015; Mathersul, McDonald, Rushby, 2013; S. White, Hill, Winston, Frith, 2006) and their impressions show the usual influence of emotional expressions (Caulfield, Ewing, Burton, Avard, Rhodes, 2014) Critically, however, they do not appear to use these impressions to guide social decisions. In an age appropriate Trust Game, autistic boys did not use facial impressions of trustworthiness to guide their decisions, despite forming similar facial impressions of trustworthiness to neurotypical children when directly asked to do so (Ewing et al. 2015) Ewing and colleagues (2015) proposed that autistic children show a functional dissociation between forming facial impressions of trustworthiness when explicitly asked to do so and implicitly using these impressions to guide behaviour. Interestingly, if facial impressions are not especially accurate predictors of actual trustworthiness (Rule et al. 2013) failure to use them to guide trust decisions may actually represent highly rational social behaviour. Current Study Here, we ask whether neurotypical adults with high levels of behavioural traits associated with autism show the same functional dissociation between forming explicit facial impressions of trustworthiness and using these impressions when making trust decisions, as AUTISTIC TRAITS PREDICT TRUST DECISIONS 7 found in autistic children (Ewing et al. 2015) If autistic traits exist on a continuum (BaronCohen, Wheelwright, Hill, et al. 2001) then the functional dissociation observed in autism may also influence behaviour to varying, milder degrees in the general population. If so, this finding would pinpoint an important source of observer variation in the use of facial trustworthiness impressions. Individual differences in autistic traits were measured using the Autism Spectrum Quotient (AQ) a well established self report questionnaire (Baron Cohen, Wheelwright, Hill, et al. 2001) Typicality of facial impressions of trustworthiness was measured by comparing participants' ratings of facial trustworthiness to those from an independent group of raters, so that each participant's score reflected their agreement with typical trustworthiness impressions (following Mathersul et al. 2013; S. White et al. 2006; Zebrowitz, Franklin Jr, Hillman, Boc, 2013) Appearance based trust decisions were measured using a modified Trust Game based on previous research (Chang et al. 2010; Ewing et al. 2015; Rezlescu, Duchaine, Olivola, Chater, 2012; van't Wout Sanfey, 2008) In the Trust Game, participants had to decide how much money to transfer to virtual 'partners' that appeared trustworthy or untrustworthy. Appearance based trust was defined as the money transferred to trustworthy appearing minus untrustworthy appearing partners (following Chang et al. 2010) Facial impressions were not accurate predictors of partner behaviour, so that the most rational strategy was to ignore them and rely only on one's personal experience with partner behaviour. We also examined the effect of social experience by manipulating trustworthy or untrustworthy partner behaviour during the Trust Game (i.e. return on investments) in addition to appearance (following Chang et al. 2010) Participants played multiple turns with the same partner, allowing them to experience how trustworthy their partner's behaviour was. Trustworthy partners transferred a fair share of money back and untrustworthy partners unfairly kept the money. AUTISTIC TRAITS PREDICT TRUST DECISIONS 8 Participants' experience based trust decisions were defined as the money transferred to the fair minus the unfair partners (following Chang et al. 2010) We additionally included a brief IQ measure