Building on a partner-switching mechanism, we experimentally test two theories that posit different reasons why promises breed trust and cooperation. The expectation-based explanation (EBE) operates via belief-dependent guilt aversion, while the commitment-based explanation (CBE) suggests that promises offer commitment power via a (belief-independent) preference to keep one’s word. Previous research performed a similar test, which we argue should be interpreted as concerning informal agreements rather than (unilateral) promises.