Supplementary Material. Perceived Agreeableness. We also conducted exploratory analyses on potential

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1 Supplementary Material Perceived Agreeableness. We also conducted exploratory analyses on potential associations of variation on OXTR and participants perceptions of their partner s agreeableness from the Big Five personality taxonomy (as in Tabak et al., 2012). Following round 7 and round 18, participants were also prompted to rate their partner s agreeableness on four descriptors (sympathetic, considerate, cooperative, and fair and just) drawn from the NEO-FFI (Costa & McCrae, 1992), the Big Five Inventory (John et al., 1991), and research investigating personality traits among potential relationship partners (Cottrell et al., 2007). Descriptors were rated on a 7-point scale (1 = not at all, 7 = extremely). Randomly ordered distracter adjectives were also included. The four descriptors had high internal consistency on both measurement occasions (αs =.87 and.91). Initial rates of perceptions of partner agreeableness were high (M = 5.69, SD = 1.19), further confirming that the computer program s initial strategy successfully elicited participants trust. No nominal single or multi-marker associations were found. Modeling Behavioral Variables as Counts. To determine which type of regression best suited the behavioral count variables, we conducted a series of Vuong tests in R version (R Development Core Team, 2005) that compared the fit of Poisson, zero-inflated Poisson, negative binomial, and zero-inflated negative binomial regression models. Vuong tests confirmed that the most appropriate type of analysis would be zero-inflated negative binomial regressions.

2 Test of the Indirect Effects. The indirect effects were calculated in R version (R Development Core Team, 2005) using package msm (version 0.7.4). Use of Deception. Although deception is generally proscribed in experimental economics (Hertwig & Ortmann, 2001), we believe that its use here is warranted (Bonetti, 1998; Cook & Yamagishi, 2008; Kimmel, 1998) and that the use of deception did not adversely influence our findings. Our experimental design relied on the establishment and subsequent breach of trust. Thus, it would not have been possible to maintain consistency in participants responses during the cooperation and defection regimes without the experimental precision provided by the use of deception (i.e., the pattern of play implemented by the computer strategy). As noted in previous research (Sanfey et al., 2003), if participants had been aware that they were being deceived (i.e., that they were not playing against another human partner), it is likely that the influence of deception would have dampened participants responses (particularly their emotional and perceptual responses since earning points was based entirely on their behavioral choices). Yet, paired samples t-tests demonstrate that post-betrayal ratings of satisfaction (M = 3.2, SD = 1.2) were significantly lower than initial ratings of satisfaction (M = 5.51, SD = 1.24); t(164) = -16.5, p <.001, post-betrayal ratings of perceived agreeableness (M = 2.42, SD = 1.18) were significantly lower than initial ratings of perceived agreeableness (M = 5.69, SD = 1.19); t(164) = , p <.001, and ratings of post-betrayal anger (M = 3.97, SD = 1.48) were significantly higher than initial ratings of anger (M = 1.72, SD = 1.12); t(164) = 15.12, p <.001. These effects suggest that the deception was believable.

3 Validity of debriefing responses. Upon completion of the experiment, it was explained to all participants that it was important to be as open and honest as possible during the debriefing and that they would not lose or gain anything based on their responses. As a result, we do not believe that participants felt coerced into stating that they did not feel as if they had been deceived for fear of losing their earnings. In addition, all participants read and signed informed consents that explicitly detailed that the amount of money that participants could earn would be based on the decisions they made during the task. Possible contamination of the subject pool. Some researchers (e.g., Jamison et al., 2008) have suggested that participants in subject pools who engage in experiments with deception may respond differently as participants in future experiments (i.e., they may assume that they are going to be deceived regardless of whether or not this is true). The University of Miami Introduction to Psychology subject pool maintains policies that minimize this concern. Specifically, students who have received all of their research credit (the requirement is to obtain 5 credits, which translates into 5 hours of participation) are no longer eligible to participate. In addition, many (if not the majority) of the studies that are conducted by all of the laboratories in the department of Psychology do not involve deception. As a result, it would be highly unlikely that any one participant would participate in more than one experiment that included deception. For this reason, we do not believe that Introduction to Psychology students assume that they will be deceived when participating in psychological experiments for research credit. Additional Procedural Details for Smaller Groups. To increase the believability of the game, in smaller groups participants were told: You are about to play two games with

4 another person who is either in this room or the room next door that has been networked with these computers. Within the E-prime program, we integrated a recording in which a female voice (acting as the controller of both testing rooms) first asked if room 1 is ready? A response was then provided by a male voice (acting as a research assistant facilitating the session in another testing room) by replying, all set. The female voice then asked, is room 2 ready? Research assistants in the present study were instructed to say almost ready, just reviewing the last slide. The female voice then said, room 1 is all set to go so you can begin as soon as you re ready. At this point, research assistants would complete the tutorial and proceed. Potential Sex Differences. Previous research indicates that variation on OXTR may have sex-specific effects on social processes (e.g., Israel et al., 2009; Tost et al., 2010). Chisquared tests determined that there were no significant sex differences in allele frequencies among all SNPs and haplotypes (p >.05). In addition, no significant sex differences were found in any of the dependent variables (initial defection, delay of retaliation, anger, and satisfaction). We also conducted separate regressions to examine the interaction of sex and: the Block 1 AC haplotype on satisfaction post-betrayal, the Block 1 GC haplotype on satisfaction post-betrayal, the Block 2 CT haplotype on anger post-betrayal, and the Block 2 CT haplotype on retaliation post-betrayal. None of the interaction effects achieved nominal significance (p >.05) and would therefore not maintain significance following multiple test correction.

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