bioRxiv preprint doi: https://doi.org/10.1101/2019.12.11.873208; this version posted December 12, 2019. The copyright holder for this preprint (which was not certified by peer review) is the author/funder. All rights reserved. No reuse allowed without permission. 1 Honest signalling of trustworthiness 2 3 Gilbert Roberts 4 Simonside Cottage, 6 Wingates, Morpeth, NE65 8RW, UK 5
[email protected] 6 7 Abstract 8 9 Trust can transform conflicting interests into cooperation. But how can 10 individuals know when to trust others? Here, I develop the theory that 11 reputation building may signal cooperative intent, or ‘trustworthiness’. I model 12 a simple representation of this theory in which individuals (1) optionally invest 13 in a reputation by performing costly helpful behaviour (‘signalling’); (2) 14 optionally use others’ reputations when choosing a partner; and (3) optionally 15 cooperate with that partner. In evolutionary simulations, high levels of 16 reputation building; of choosing partners based on reputation; and of 17 cooperation within partnerships emerged. Costly helping behaviour evolved 18 into an honest signal of trustworthiness when it was adaptive for cooperators, 19 relative to defectors, to invest in the long-term benefits of a reputation for 20 helping. I show using game theory that this occurs when cooperators gain 21 larger marginal benefits from investing in signalling than do defectors. This 22 happens without the usual costly signalling assumption that individuals are of 23 two ‘types’ which differ in quality. Signalling of trustworthiness may help 24 explain phenomena such as philanthropy, pro-sociality, collective action, 25 punishment, and advertising in humans and may be particularly applicable to 26 courtship in other animals.