I was always told that industrial toilet seats were U shaped so when the OSHA inspector stopped to get a drink, if the lid fell, it wouldn’t break his neck.
Well, consider my ignorance fought! Although comments like this one always tend to make it fight back. 
I take it this is the column you’re referring to?
Cecil only answered half the question. How come home toilets don’t have U shaped seats?
I thought U shaped seats were so if you were in a hurry and couldn’t wait, you could lift the seat WHILE peeing.
I’d imagine it’s because people generally don’t want their bathroom to be reminiscent of a public restroom…
Wikipedia Cite
I’m glad toilet seats have that cut-out thing in front. It could get messy when a woman is at a certain time of the month. (I knwo, ew.)
“The relevant male apparatus being in the front, this makes the front of the toilet seat (particularly the underside) pretty gross — or rather, it would make it gross, if toilet-seat makers hadn’t been shrewd enough to head the problem off at the pass.”
I have to take Cecil Adams to task on this one!
The cutout in the toilet seat is an OSHA requirement!
As we all know, toilet seats don’t always stay up. This causes 2 problems. When it falls a man must either quickly step back or try to stop it in mid fall. If he isn’t quick enough in either action he can risk traumatic injury!
This simple modification has prevented untold hundreds of workplace injuries, and thousands of lost man hours EVERY YEAR!
link to column: Why are public toilet seats always “U” shaped? - The Straight Dope
LATE EDIT: Sorry, I just realized that there was another thread on the topic, so I’ve merged the two threads.
My theory is because at certain times of the month, women are more likely to leave blood on the front of the seat. It isn’t just men who are filthy.
I would have thought this was completely made up, but perhaps not. From here:
If that’s to be believed, maybe it did start there, and now is just continuing through inertia.
No. In this, as in many, many other cases, OSHA merely codified existing practice. I can personally swear that split toilet seats were normal in public facilities in the 50s, and I believe they were a good deal older.
That column has always bugged me, because it never addresses the initial, and to me more interesting, question: Why are residential toilet seats always round? He give some fine reasons for U shaped toilet seats in public…are men not pigs at home, too?*
*Note from the woman who cleans the toilets at home: yes, yes they are.
In the interest of science, I just did an experiment. If the lid were to fall and I failed to step back, it would not hit me at all unless I stood with my legs touching the front of the bowl. Even then, it would catch me just above the knees, and just barely.
Which came first? O-shaped toilet seats or U-shaped ones?
I think that’s fairly obvious - the gap makes the toilet seat structurally weaker than a fully circular one, and places extra strain on the hinge and attachment at the rear.. In a public toilet, the balance between strength and hygiene is shifted away from strength by contact with the buttocks of the general public; in a residential toilet, hygiene is less of a concern, as one’s own toilet seat may be maintained to one’s own standards (or lack thereof).
That said, I’m not satisfied that the IAPMO cite given is the truth. While IAPMO was founded for the purpose of code development in 1926, there’s no evidence available to the general public that this particular standard was adopted before open-front toilet seats became common (and patent history suggests the opposite), and no evidence that women or wiping were in the minds of the inventors - instead, patents speak generally of hygiene (and also of “water closets”).