Why do we trust each other? We carried out three experiments to test whether interpersonal trust depends on perceived self–other overlap. As previous studies suggest that enfacing (feeling ownership for, and include more into oneself of the face of) an avatar might make one trust this avatar more, we exposed participants to faces of ingroup and outgroup avatars that moved in synchrony or out of synchrony with the participant’s own facial movements, and assessed the impact of synchrony on self–other overlap and trust measures. Experiment 1 used ingroup faces and successfully showed that synchrony (manipulated within-participants) increased self–other overlap and trust, which we assessed by means of the Trust Game and the Implicit Association Test (IAT). In Experiment 2, which used outgroup faces and a within-participants design, synchrony still increased scores in the Trust Game but the IAT was no longer affected. Experiment 3 replicated Experiment 2 but with synchrony varying between participants, which eliminated the synchrony effect in both trust measures. Importantly, Inclusion of Other in the Self (IOS) ratings were found to predict the IAT effect in synchronous conditions. Taken altogether, our findings suggest that interpersonal trust is mainly driven by perceived self–other overlap. Besides group identification, appearance, and voluntary movement, synchrony is just one of several sources contributing to perceived self–other overlap.