The psychophysiological responses of 60 subjects were measured as they observed a performer play a roulette game. Half of the subjects were led to believe that they were similar to the performer in personality and values, and half were led to believe that they were dissimilar. Half of the subjects in each condition believed that the performer won money and experienced pain as he played the game, and half believed that he performed a cognitive and motor skill task. Subjects who observed a performer who ostensibly experienced pleasure and pain exhibited greater psychophysiological reactions than subjects who did not. Subjects who believed they were similar to the performer tended to react more strongly than subjects who believed they were different from him. Similar subjects also reported identifying most with the performer and feeling the worst while he waited to receive shocks. It was concluded that the similar subjects empathized most with the performer who appeared to experience pleasure and pain. When required to make a choice between helping themselves at a cost to the performer or helping the performer at a cost to themselves, the subjects who reacted most empathically behaved most altruistically. The results were interpreted as casting some light on century-old questions about the human capacity for altruism.