Any document that was written during the event by someone who witnessed the event, such as a document on the Battle of Gettysburg, a primary source document would be by someone who was at the battle, and could see what was happening first hand.
The answer is never because GDP per capita depends on GDP and population. Countries with more people have the potential to make more goods and services. But, having more people also lowers GDP per capita, because more people have to share these products.
