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Several aspects of the inverted U-model regarding the relation between activation and performance were tested in an experiment in which activation was manipulated both by increasing metabolic demands and by varying psychological demands. Psychological stress influenced performance but the direct manipulation of activation by increasing physical stress had no effect on performance. From these results we conclude that it is very unlikely that activation is causally related to performance. A better explanation seems that a stressor influences both activation and performance and that the effect of a stressor is highly specific and depends on the kind of stressor and the kind of task.