Ethics and Altruism Altruism in humans: an evolutionary approach

Altruism was originally defi ned as a cooperative act infl icting a cost to the donor and only giving a benefi t to the recipient. However, no study has ever conclusively demonstrated the existence of altruistic behaviours that do not give any short-or long-term benefi ts to donors. Th erefore, the term altruism is generally used for costly behaviours that may potentially also be benefi cial for the donor. In accordance with this broader defi nition, two main concepts have been traditionally proposed to explain the evolution of altruism: inclusive fi tness (Hamilton, 1964) and reciprocal altruism (Trivers, 1971). Altruism among kin evolves through kin selection (Hamilton, 1964), as donors may gain inclusive fi tness benefi ts by helping their relatives. Regardless to the genetic relationship between two interactants, reciprocal altruism may become evolutionarily stable when individuals alternatively or simultaneously act as a donor or recipient of the altruistic behaviour, thus balancing the costs and benefi ts of the exchange over time. Since Hamilton and Trivers seminal papers, additional theories or factors favouring altruism have been proposed (reviewed in Kappeler & van Schaik, 2006; Nowak 2006), including indirect reciprocity, multi-level selection, and reputation. Th e concept of indirect reciprocity applies to interactions where the donor of the altruistic act may receive benefi ts from the former recipient diff erent from those given, or where the benefi t is returned by a third party. Reputation may be one of such benefi ts: individuals may act altruistically (e.g. through public donations to charities) to improve their reputation within the group and/ or to signal potential mates or competitors that they may successfully aff ord the cost of altruism (Milinski et al., 2002; Roberts, 1998). Multi-level selection suggests that groups composed of all altruistic members should be more cohesive and have a greater average fi tness than groups composed of a mixture of altruists and defectors (i.e. individuals who rarely or never act altruistically despite benefi ting from the altruism of others) and their proportion should thus increase within a population over time. All the theories proposed so far share the same prerequisites to reduce the risk of defection and for altruism to become evolutionarily stable: animals need to recognize and repeatedly interact with the same subjects over a given period of time, remember the type of interactions they had in the past, and modify their behaviour accordingly. Th ere is increasing evidence …