When you do something very unusual, you describe the reasons in terms of something unusual about the situation. When you watch someone else do something equally unusual, you are likely to think the other person is "mentally ill." This is an example of the

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This is an example of the actor-observer effect, which refers to the tendency for individuals to attribute their own behavior to situational factors, but the behavior of others to dispositional factors (such as their personality or mental health). This could lead someone to label someone else's unusual behavior as a symptom of mental illness, whereas they would justify their own similar behavior as a response to a unique situation.

When You Do Something Very Unusual, You Describe the Reasons