Afight-or-flight response is to humans because it

The fight or flight response results in certain physiological responses that would be of benefit in a situation of life or death.  The neural response results in the adrenal gland releasing the hormone epinephrine. This results in the release of additional energy, which provides a temporary energy boost. Other changes to facilitate violent muscle reactions include:  heart acceleration, diversion of blood to the most important organs, liberation of excess energy, and dilation of blood vessels for muscles. It is arguable that in a situation of violence, such as self defence, previous exposure to controlled violence, such as martial arts training, would be of more benefit, as these physiological changes can be controlled and channeled for the best results. 

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