Expert: John Shea - 7/10/2006

Question
A former soldier I was talking to last week said he collapsed when a bullet missed his head by a fraction of an inch (it went through his hair). "My legs turned to jelly" he said. His experience seems to be quite common - faced with great danger, people often find that, far from finding an adrenalin-driven 'super-human strength', their legs too, turn to jelly, rendering them incapable of defending themselves.
How is it that such an apparently counter-productive response has evolved?

Answer
Dear Michael
This is kind of peripheral to my areas of expertise, so you should try one of the psychology experts. I read something about this phenomenon in a popular science journal a couple of weeks ago.
The reaction the soldier describes is a common one.  A lot of people in accidents experience a kind of temporary paralysis.  What is probably happening is several different neurotransmitters are being released rapidly.  One, of course, is adrenalin, but there are others as well that may cancel out the "flight/fight" response.  You are correct that a prolonged delay in reaction to sudden stresss would be very maladaptive, but a brief delay -long enough for the person involved to gather their wits- might be advantageous.
It is probably not the case that this is an adaptive response that has evolved through positive selection for it.  Rather this is probably a maladaptive response that is not yet strongly enough selected against for it to be weeded out from the living human genome.  The particular circumstance the soldier describes, a sudden threat from an unseen, unknown source (the shock wave of the bullet passing so close to his head) may be such an evolutionary novelty. It is also (thankfully) not something that a plurality of the humans living in the world today experience on a regular basis.  So, any selective pressure against this shock paralysis is going to be very much concentrated on a small subset of many societies (e.g., soldiers, police) rather than on the population in general.
Again, this touches on an area in which there is more to be learned from psychologists than anthropologists.  If you Google search "flight/fight" and "biopsychology" you might find some useful links online.  As with many such things, though, having a look at an introductory college psychology textbook might get you to an authoritative source on this.
Sincerely
John Shea