the grudgers of Chapter 10. These were birds that helped each other in an apparently altruistic way, but refused to help—bore a grudge against—individuals that had previously refused to help them. Grudgers came to dominate the population because they passed on more genes to future generations than either suckers (who helped others indiscriminately and were exploited) or cheats (who tried ruthlessly to exploit everybody and ended up doing each other down). The story of the grudgers illustrated an important general principle, which Robert Trivers called ‘reciprocal altruism’.
