Members of the royalty, for centuries, would only marry members of other royal families. Royal marriages were often seen as strategic politically and used to help build alliances in other nations. Many members of the British royal family also married their cousins, though practice ended during the 20th century.

In fact, no member of the British royal family married outside of royalty until 1937—when Edward, Duke of Windsor, the uncle to Queen Elizabeth II, who briefly reigned as King Edward VIII, married American divorcee Wallis Simpson. The marriage, shrouded in controversy at the time, forced him to abdicate the throne, and as a result, he served as king for less than one year in 1936.

Since then, it has become more common for royals to marry outside of royal lines, but Miller tweeted Monday that he views this as a “longterm concern” for the monarchy, as it may end the sense of mystery that keeps millions of people across the world intrigued by the royal family.

“Key to monarchy is its mystery. Key to its mystery is that monarchs descend from an ancient line of fabled kings & queens,” he wrote.“Though it may not be apparent now, a longterm concern for UK monarchy will be if, due to marriages, future monarchs have same family trees as their subjects.”

He continued in a follow-up tweet: “Any system based on heredity privilege—whether in UK, India or anywhere—historically married within its ‘social rank’ to justify said privilege. Eg, if a Duke’s family were not of ’noble’ birth (a notion rejected of course in US) why would he get title/castle but not townspeople?”

The responses to Miller’s thoughts included people who pointed out that British royals used to marry their cousins to avoid marrying people who were not royal.

“The mystery was incest Stephen,” tweeted journalist Ken Klippenstein.

“Ah yes, inbreeding famously worked out well for European royalty,” tweeted Mother Jones editor Clara Jeffery.

Royal Marry Commoners More Often Than Ever

After Prince Edward was forced to abdicate the throne over his marriage, no other member of the British royal family would do so until 1960—more than 30 years later—when Princess Margaret, Countess of Snowdon, married British photographer Antony Armstrong-Jones.

However, in recent years, British royals have more frequently married people who are not members of any royal family. In 2005, King Charles III, who became king last week following his mother’s death, married Camilla Parker Bowles, who became known as Camilla, Queen Consort after Charles took the throne.

The 2010s saw two high-profile royal marriages to commoners. In 2011, Prince William, who is expected to be king after Charles, married Catherine Middleton. In 2018, Prince Harry married Meghan Markle, and several other members of the British royal family have also married commoners in recent years

Newsweek has reached out to America First Legal, the legal foundation founded by Miller, for comment.