Any idea why so many words and phrases referring to a certain rigidity of personality, spring from roots associated with the same area of the human anatomy? I mean, of course:

costive
constipated
anal retentive
rectitude

slightly off-track:

fundamental

plus, of course, the other favoured use:

ers-hole (in Chaucerian spelling)

But it's the first group I am most intrigued about. And no, this is not meant to be a scatological thread.