We trust strangers every day. Deciding to trust another depends on the social and legal environments that influence calculative trust as well as unconsciously processed neural signals. Unenforceable communication (“cheap talk”) has been shown to increase cooperative behaviors, but why it does this is not well-understood. Herein we test whether physiologic synchrony during pre-decision communication explains why one would trust a stranger when the money at risk is meaningful. Working-age adults discussed how to share $480 before making sequential decisions in private. The social interaction was structured so that the first decision-maker had to sacrifice money s/he controlled by sending it to the second decision-maker in order to grow the money at stake. Electrodermal activity was captured during communication and two measures of synchrony, intersubject correlation and dynamic time warping, were evaluated for their predictive accuracy. We found that the dynamic time warping measure of synchrony predicted trust with a stranger with 60% accuracy, a 15% improvement over the accuracy using intersubject correlation. Our findings demonstrate that dynamic time warping is a more effective measure of physiologic synchrony, leading to new insights into the mechanisms through which communication influences the decision to trust a stranger.