Why is it that when a man is standing at a urinal urinating, he indulges himself in the urge to expectorate? I do it, I'll bet even you do it! I've seen it lots of times.
-- JF, San Diego
I considered not answering this for a year or two so I wouldn't disturb Joe's picture of M.A. as a useless idler. But instead we took up the challenge, and luckily, among our cavalcade of friends is a La Jolla M.D. whose specialty happens to be urological dysfunctions of the rich and famous. At my urging, he checked the current literature, consulted with his peers (sorry, bad urologist's pun), and determined that there is no physiological connection between micturation and expectoration. At least none that anyone's taken the time to prove. You can pee without spitting, no problem. And, luckily, you can spit without peeing. This is the sum total of scientific information available on the subject.
Now we're reduced to quasi-scientific speculation and creative imagineering, which I'm particularly good at. Just enough facts to make my case look reasonable to the average citizen, but fuzzy enough around the edges to allow me to weasel out of it without losing face, should I be challenged. What more could a know-it-all ask for?
Theory number one has to do with the "personal space" that each of us has around our bodies-- the psychologically bounded territory that we consider inviolable under most circumstances. Americans, as a general rule, have a two-foot air space around them that they protect from invasion through subtle body-language cues. According to one psychologist, spitting is, among other things, meant to repel people and keep them at a distance. (Don't snicker. For this he needed six years of postgraduate work.) A man standing at a urinal might feel vulnerable, and spitting could be a way of enforcing his personal boundaries.
If that theory is a little too anthro-psycho-squishy for your taste, how about theory number two. Spitting when you pee is a conditioned reflex. You started doing it when you were little, either by watching Dad or because of your little-kid fascination with body parts and functions, especially things that come out of your body that you can control. The spit-pee connection, according to this theory, is no different from Pavlov's salivating dogs.