(a) Mean probability of acceptance across all participants. The probability of acceptance increased as a function of the magnitude of benefits for the other and decreased as a function of the magnitude of costs to oneself. The benefits for the other and costs to oneself varied independently and pseudorandomly from 3 to 25 Swiss francs. (b) The participants in the experimental group, who had committed to spending the money on others, showed significantly more generous behaviour than the control group, who had committed to spending money on themselves. Generous behaviour was defined as the probability of accepting the option presented (t(46)=2.02; P<0.05). Error bars are s.e.m. (c) The participants in the experimental group showed a greater increase in happiness (Happiness(T2)-Happiness(T1)) than the control group did (t(46)=1.87; P<0.05). Error bars are s.e.m.