Why do people cooperate in some interactions but not others when incentives are identical? Economic models focus on game structure, while psychological models emphasise stable traits. We integrate these perspectives by conceptualising personality expression as a strategic signalling process in which individuals selectively express trait-relevant states to influence others’ beliefs and behaviour. We formalise cooperation as a sender-receiver game in which one party holds private information about prosociality and may signal it, while the receiver updates beliefs and chooses whether to cooperate. We test this framework in two economic-game studies. Study 1 (N = 320; online) identified personality items perceived as informative in cooperative contexts. Participants rated prosocial items as more useful than extraversion items for guiding behaviour, especially in the Trust Game, and adjusted hypothetical giving and expectations accordingly. Study 2 (N = 250; preregistered lab experiment) tested whether individuals incur costs to signal personality and whether signals influence behaviour. Participants paid more to signal prosocial than non-prosocial traits, and receivers gave more to higher-prosocial partners. Prosocial signals modestly increased efficiency in the Trust Game and reduced inequality in the Dictator Game. These results show that personality operates as an economically meaningful signal that shapes beliefs and cooperation.