People do more for their fellows than return favors and punish cheaters. They often perform generous acts without the slightest hope for payback ranging from leaving a tip in a restaurant they will never visit again to throwing themselves on a live grenade to save their brothers in arms. [Robert] Trivers together with the economists Robert Frank and Jack Hirshleifer has pointed out that pure magnanimity can evolve in an environment of people seeking to discriminate fair weather friends from loyal allies. Signs of heartfelt loyalty and generosity serve as guarantors of one s promises reducing a partner s worry that you will default on them. The best way to convince a skeptic that you are trustworthy and generous is to be trustworthy and generous.