Converging data suggest that human facial behavior has an evolutionary basis. Combining these data with Seligman's preparedness theory, it was predicted that facial expressions of anger should be more readily associated with aversive events than should expressions of happiness. Two experiments involving differential electrodermal conditioning to pictures of faces, with electric shock as the unconditioned stimulus, were performed. In the first experiment, the subjects were exposed to two pictures of the same person, one with an angry and one with a happy expression. For half of the subjects, the shock followed the angry face, and for the other half, it followed the happy face. In the second experiment, three groups of subjects differentiated between pictures of male and female faces, both showing angry, neutral, and happy expressions. Responses to angry conditioned stimuli showed significant resistance to extinction in both experiments, with a larger effect in Experiment 2. Responses to happy or neutral conditioned stimuli, on the other hand, extinguished immediately when the shock was withheld. The results are related to conditioning to phobic stimuli and to the preparedness theory.