The human body’s physical “fight or flight” response evolved to help it evade a dangerous situation, which historically involved extreme physical exertion. The rush of steroids into the bloodstream essentially turns off unnecessary systems, including some higher thinking processes, to aid in escape.
When we make statements like this proclaiming we understand the complex workings of things like human physiology I always have to shake my head. We may be smart, us programmers and designers, but let's not pretend to understand the order and meaning of everything. And if you do understand and you have the means, provide some kind of reference to where the claim originates from so we can all learn more.
* edited because I sounded like a jerk and wanted to make my point clearer