Browsing through the reports listed on the Paranormal Database, I was struck by the number of headless ghosts reported there. There's one exception - a bodiless head in Aberaeron, Wales - but on the whole there are far more bodies than heads around.
Now, being beheaded is about as traumatic a way to die as I can imagine. It would be the most sudden of sudden deaths, and the spirit could be forgiven for being mightily confused when it finds itself dropped into an afterlife for which it has had no time to prepare. Therefore, beheadings are likely to produce ghosts. But why no head? There's nothing to suggest that the spirit-head is lost when the physical head goes.
In thinking about this, I tried to put myself in the situation of someone who died without preparation. Someone who lost their physical presence and found themselves in a new form, a form with which they were wholly unfamiliar.
In that situation, I'd try to appear to someone else to ask what the hell was going on. Close your eyes and try it. Imagine you have no physical form and you have to reconstruct yourself from scratch. You're inexperienced so you'll try to form this image around yourself.
What's the hardest part to visualise? You can't see your back, but you can extrapolate a general back-shape from those you've seen in the past. One thing you can't extrapolate from others is your own face.
The hardest part of such a manifestation is the head. It's the one thing you can't see without a mirror, the most difficult and detailed part to reproduce.
Maybe headless ghosts have heads, but just can't work out how to make them appear. With practice, they'll learn to do it - but the range of intelligence in humans is, I'm certain, reflected in the dead. If you're a dope now, you won't become a great thinker when you die. So some will never learn how to manifest a whole image. Some never work out how to manifest at all.
I don't think we can assume that every headless ghost represents a beheading. It's hard to tell who it is, with no head, so assuming it's some famous historical figure who ended their lives eight inches shorter is, perhaps, presumptuous.
It might equally be their gormless brother or sister, a ghost who does, in fact, have a head, but one which has never been contaminated by intelligence.
If that's the case, it might be a blessing we can't see their faces.