Supplementary material from "Pupil-mimicry conditions trust in partners: moderation by oxytocin and group membership"

Using incentivized trust games with partners from ingroup and outgroup whose pupils dilated, remained static, or constricted, this study replicates our earlier findings. It further reveals that (i) male participants withhold trust from partners with constricting pupils and extend trust to partners with dilating pupils, especially when given oxytocin rather than placebo; (ii) female participants trust partners with dilating pupils most, but this effect is blunted under oxytocin; (iii) under oxytocin rather than placebo, pupil dilation mimicry is weaker and pupil constriction mimicry stronger and; (iv) the link between pupil constriction mimicry and distrust observed under placebo disappears under oxytocin.

We suggest that pupil-contingent trust is parochial and evolved in social species in and because of group life.