The fight or flight response, also called the "acute stress response", was first described by Walter Cannon in the year of 1929. The theory states that animals react to threats with a general discharge of the sympathetic nervous system. The response was later recognized as the first stage of a general adaptation syndrome that regulates stress responses among vertebrates and other organisms. In layman's terms, an animal has two options when faced with danger. They can either face the threat ("fight"), or they can avoid the threat ("flight").
http://en.wikipedia.org/wiki/Fight-or-flight
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