Princess Diana, Kate Middleton and any woman marrying into the royal family had to prove themselves during their first visit to Balmoral. In Paul Burrell's memoir, The Royal Insider, he claimed the tradition is used to see whether outsiders can adjust to life within The Firm. "The 'Balmoral Test' was cruel. Outsiders, most importantly potential brides, were brought there like specimens to be inspected by the family to see if they came up to scratch and if they were suitable," he wrote in a passage of the book obtained by Fox News Digital.