Why do promises affect trustworthiness, or do they?

We set out to test whether the effect of promises on trustworthiness derives from the fact that they are made (internal consistency) or that they are received (social obligation). The results of an experimental trust game appeared at first to support the former mechanism. Even when trustee messages are not delivered to trustors, trustees who make a promise are more likely to act trustworthy than those who do not make a promise. However, we subsequently ran a control treatment with restricted (non-promise) communication to examine whether the correlation between promises and trustworthiness is causal. The results show that the absence of promises does not decrease average cooperation rates. This indicates that promises do not induce trustworthiness, they are just more likely to be sent by cooperators than by non-cooperators.