USA, UK, Australia, Ireland, New Zealand, Canada, etc… Why not take an advanced English grammar teacher from each of those countries, lock them up in a room, and tell them the only way to unlock that door is to agree on this topic?

Obviously, as what's correct differs vastly between these countries, such a move should hopefully result in proper documentation outlining not only what's correct in each country, but also details how the grammar on this differs between the countries.