First, a moment of Zen:
This is one of the great mysteries of life. All I know is when I take a gigantic, um, trip to the bathroom, I feel empty, yet, somehow, fulfilled.
Thought you might be interested in a classic story that explored that very topic, from the perspective of athletes. It's a terrific read, and I'm not just saying that because I work at the magazine that published it. Nut graph: "We can exhaustively explore every aspect of athletic life — victory, defeat, violence, racism, drugs, brain damage, paralysis, death — but nothing reveals as much about the physiology, psychology and sociology of sport as the excretory experience of athletes."
Another shifts gears:
I am not a psychologist but rather a pediatric urologist at Wake Forest University, and as someone who deals with poop every day, all day, I can tell you that the psychology of pooping is indeed worthy of serious study.
I treat countless children for bladder problems – bedwetting, accidents, urinary frequency, among others – that are directly related to the chronic holding of poop. Masses of poop build up in the rectum, pressing against the bladder and irritating the bladder nerves. One of my patients was the Arlington, Virginia girl who was kicked out of preschool for having "too many" accidents. Her doctor and her pediatric urology clinic failed to detect the Nerf-basketball-sized mass of stool lodged in her rectum. I see this type of case daily.
As I explained in recent posts for the New York Times and the Huffington Post, poop-holding has become epidemic among children in our culture, and kids are suffering for it in ways that even most pediatricians do not recognize. Why are kids holding it? Three reasons: 1.) the terrible diet we feed our kids, 2.) toilet training them too early (a two-year-old does not have the wisdom to understand the importance of emptying one's bowels in a timely manner), and 3.) the horrific bathroom conditions in the vast majority of our kids' schools.
We make it very difficult for our kids to feel comfortable pooping. And then we mock people who are trying to do something about it. (After my Times piece was posted, Gawker promptly belittled it here.) If anyone is interested in the connection between constipation and bladder problems, the research – my own and that of others – is here.