Humans are characterized by an unusual level of prosociality. Despite this, considerable indirect evidence suggests that biological kinship plays an important role in altruistic behaviour. All previous reports of the influence of kin selection on human altruism have, however, used correlational (rather than experimental) designs, or imposed only a hypothetical or negligible time cost on participants. Since these research designs fail either to control for confounding variables or to meet the criteria required as a test of Hamilton's rule for kin selection (that the altruist pays a true cost), they fail to establish unequivocally whether kin selection plays a role. We show that individuals ...

<1.0) or r = .25 vs. r = .125 (F(1, 63) = 2.00, MSE = 0.06, p><1.0). Participants were no more willing to invest in their best-friend than r = .125 (F(1, 190) = 1.94, MSE = 0.05, p><1.0), and r = .125 vs. charity (F(1, 17) = 1.71, MSE = 0.07, p><1.0), or between r = .25 vs. r = .125 (F(1, 68) = 1.62, MSE = 0.06, p>