PART I
In many movies, from James Bond to Signs, we've seen our heroes lie in hiding, waiting endless hours while some known harm lurked about. We've all experienced something similar while playing hide-and-seek, or waiting for a practical joke to fall on our victim, or waiting for our parents to retreat into their bedroom so we know its safe to come home three hours past curfew.
But during these times of intense concentration, with our hearts pumping and our bodies on the edge, something happens to us that never happens to 007: we need to pee. Urinate. Micturate. Whatever you call it, one of the main functions of adrenaline is to expulse any excess urine from the urinary bladder, the major and minor calyx, and the ureters.
Why? Well, urine is 99.999% water, and water isn't exactly light; so when your body thinks it needs all its physical capabilities, it tries to lighten the load. And if you need your body to run its fastest, the next most important thing beyond weight reduction is getting oxygen to your muscles. The very first body system your sympathetic nervous system is going to steal blood from is your digestive tract.
So maybe you're frightened by a lurking assailant, or maybe you have to dash into your room before your parents decide to check your bed, or maybe a co-worker walking in on you while you're in the bathroom scares the hell out of you. Whatever causes it, when you experience an adrenaline rush, it means that a) you pee really well, but b) your body's resources will have been redirected, and you can't poop worth a damn.
PART II
But that leads to the next question: food and food by-products weigh a lot too -- so why don't they come out?
You don't poo yourself because, more significantly than being residual weight, the foodstuffs in your alimentary canal can be oxidized to provide energy that a frightened person might very well need.
A case study:
A 32-year-old male is confronted by an armed assailant. First impulse to the amygdala: Fight, Flight or Fright. From the physiological sense, the least expensive route for the body to take is Fright. Adrenal glands squeeze, releasing adrenaline and epinephrine into the common circulation, making the heart beat fast and the lungs breathe fast, and shutting down any non-essential functions -- including poo-poo processing. While urine is expelled, feces is kept in check -- the body needs the energy.
Poo is not quite poo until it is committed to the rectum. Since all the energy in the body is being harnessed to run, digestion is halted. Screw hematopoesis, screw spermato, screw oogenisis -- there will be time for blood production and reproduction down the road (hopefully). Right now, your amygdala is saying, "Get the HELL out of there!" Blood is redirected from the digestive tract, delivering much needed oxygen to the skeletal muscles. As such, digestion stops. No poop formation. But more: no poop expulsion. Your body has redirected the energy it takes to push out a poop.
But that's not all. Beyond saving energy, there is the fact that poo has a distinctive smell (in case you didn't know). If you poop yourself while running from a saber-toothed tiger, the beast will be able to smell your location that much easier. And while it's rare to be attacked by a saber-toothed tiger any more, we do have physical and psychological threats to deal with -- taxes, terrorists, and, for many of us, interruption in the bathroom.
So you think that there's something wrong with you because you're a Shameful Shitter? Here's some news for you: you're normal.
When you're on the toilet, you're vulnerable. If they had toilets in caveman times, that's where the saber-toothed tigers would hang out when they were hungry. So your Shameful Shitting is really just your evolutionary response to a threat. When you feel threatened, you revert back to your caveman instincts.
You don't poo when you scared, because the smell is distinctive -- you don't want a saber-toothed tiger following your poo smell back to your cave.
You don't poo when you're scared, because you might need that energy to run or to fight.
You don't poo when you're scared, because, well...
...because you're a pussy, and you don't want others to know you're there.
But that's good -- if it was a saber-toothed tiger entering the bathroom instead of your coworker, the Shameless Shitter who goes on with his business might as well hold up a sign saying "free lunch in stall #2."
The smell will lead anything with a nose right to you.
Different humans are scared of different things. Not everyone is threatened by the sound of another person entering the bathroom. But those that are aren't necessarily unenlightened slaves to archaic Victorian taboos. They are more in touch with their animal instincts -- and if there were a nuclear war and the survivors were hunted by zombies with hyperactive noses, the Shameful Shitter would be the ones more likely to survive.
-- Dave J [1]