We study the evolution of preferences in games of voluntary contributions to public goods. All contributors incur identical fitness cost but experience subjectively different costs of contributing. Given their preferences, individuals behave rationally. A contributor is always disadvantaged in fitness terms vis-a-vis non-contributors in any given interaction. Nevertheless, because a non-contributor runs the risk of ending up in a group in which the public good is not provided, altruists, whose subjective costs are lower than fitness cost, may have above average fitness. This guarantees the survival of altruists in the long run.