The International Association of Plumbing and Mechanical Officials requires U-shaped toilet seats in public restrooms, and there's nothing to stop a homeowner from installing one in a home bathroom. Many theories concerning this requirement exist; according to association representative Lynne Simnick, they were invented mainly for women.
Reasons for a Split Seat
One theory concerning the origin of the U-shaped toilet seat is that it's a safeguard against unsanitary male toilet habits. Eliminating the front part of the seat eliminates the possibility of unseen dribbles on the underside. Another theory is that a U-shaped seat is easier to clean because it has less surface area. While both are points in favor of split seats, Simnick says that the main reason why the international association adopted the split-seat requirement was to make it easier for women to clean themselves hygienically without coming into contact with the seat. The association's requirement isn't a law, but many communities follow it.