The myth that Anne had six fingers originated soon after her death. It was never mentioned during her lifetime, and her enemies - who criticized her for every flaw - would have been sure to make note of a sixth finger. Deformities of any kind were seen as the mark of the devil.
Anne would never have been accepted as one of Katharine of Aragon's ladies in waiting if she'd had even the "small bit of nail" some authors and historians have mentioned. Competition for these positions was fierce. Even a small imperfection could disqualify a candidate.
Nor would Henry VIII have considered her a suitable mother for his children if she had a physical flaw. He might not have even sought her as a mistress in that case.
She could not have hidden such a deformity with swinging sleeves, or by tucking it behind another finger as is sometimes alleged. Her servants washed her hands before every meal and saw her completely nude every day. And servants were the number one source of gossip in those days. It would have been known by the whole court within days of her arrival and it would have been made a big issue when it became clear Henry wanted her to be the next queen.
Nicholas Sander is the one who popularized the rumor of a sixth finger in his book, The Rise And Growth of the Anglican Schism, written in 1585. Sander likely never saw Anne Boleyn in person; he was only about six years old when she died. He was a fervent Catholic apologist, writing during the time of Anne's daughter, Elizabeth. Sander despised Elizabeth and her mother, whom he blamed for the start of the heresy that drove England from the Catholic faith.
As a result, Sander piled every slander and infamy he could think of on Anne Boleyn's memory. In that era, physical deformity or ugliness was thought to stem from evil inside. Since Sander saw Anne as evil, he made her hideously ugly in his descriptions.
Anne Boleyn was rather tall of stature, with black hair, and an oval face of a sallow complexion as if troubled with jaundice. She had a projecting tooth under the upper lip, and on her right hand six fingers. There was a large wen under her chin, and therefore to hide its ugliness she wore a high dress covering her throat. In this she was followed by the ladies of the court, who also wore high dresses, having before been in the habit of leaving their necks and the upper portion of their persons uncovered.I've previously written about why Sander was wrong about Anne's hair color, and a casual glance at portraits of the era shows Anne and her ladies continued to wear low-cut gowns. But the myth about the sixth finger found traction. It was so pervasive that George Wyatt, grandson of Thomas Wyatt, the poet, had to mention it in his The Life of Queen Anne Boleigne:
Wyatt never saw Anne either - and was born after his famous grandfather had died - but likely felt he had to take on a myth that kept growing and defend her in a way that felt reasonable to him, conceding that maybe one of her fingers was a bit irregular, but it wasn't as bad as people thought it was. He did the same thing with the "wen" Sander mentioned. Wyatt explained she had a few moles, though he's the first to mention she had them. He also said she had the "clearest of complexions," but her contemporaries described her as being "swarthy" or olive-skinned.
Still, myths are sometimes more fun than the truth, and so it remained as a bit of "trivia" passed down over the years. Even some modern biographers accept the Wyatt description of an extra nail on her finger, but there is not a single shred of evidence originating from her lifetime - and, again, it's something people would have mentioned.
Recently, a lifelike mannequin of Anne Boleyn was installed at Hever Castle ... they asked the artist to remove the sixth finger.