Abstract: Social organisms often need to know how much to trust others to cooperate. Organisms can expect cooperation from someone who depends on them (i.e., fitness interdependence), but how do individuals assess fitness interdependence? Here we extend fitness interdependence into a signaling context: costly helping behavior can honestly signal the strength of fitness interdependence, such that those who help are trusted more. We present a mathematical model in which agents help others based on their fitness interdependence, and recipients use that information to assess whom to trust. At equilibrium, helping is a costly signal of interdependence: helping is worthwhile for those who value the recipient (and thus will repay any trust), but is not worthwhile for those who do not value the recipient (and thus will betray the trust). Recipients demand signals when they value the signalers less and when the cost of betrayed trust is higher; signal costs are higher when signalers have more incentive to defect. Signaling systems are more likely when the trust games resemble Prisoner’s Dilemmas, Stag Hunts, or Harmony Games, and are less likely in Snowdrift Games. Furthermore, we find that honest signals need not benefit recipients, and can even occur between hostile parties. By signaling their interdependence, organisms benefit from increased trust, even when no future interactions will occur.
