William Shakespeare has long been celebrated as the greatest playwright of all time (and he's certainly the most quoted). Historians have speculated whether his name might be a pseudonym for a lesser-known writer or whether he had assistance in composing his plays, among other theories. In 2016, Oxford University Press credited Shakespeare's contemporary Christopher Marlowe as co-writer of three plays in Henry VI.

Now, new evidence has come to light that casts doubt on Shakespeare’s sole authorship, this time for Henry VIII. According to an analysis [PDF] published prior to peer review on arXiv.org, the Bard wrote roughly half of Henry VIII. His contemporary, playwright John Fletcher, wrote the rest.

The conclusion was based on the findings of an algorithm taught to examine word choice and writing style, created by researchers at the Czech Academy of Sciences in Prague.

The program first "learned" each writer’s approach to their craft by reading four plays by Fletcher and by Shakespeare, written at about the same time. The algorithm identified traits unique to each. Fletcher, for example, tended to use ye instead of you, or ‘em in place of them.

The algorithm was then applied to Henry VIII. It earmarked the first two scenes as being written by Shakespeare. Fletcher wrote the next four. The writers' styles then mixed until later in the play, when Shakespeare’s voice appeared to take hold.

Collaboration among playwrights was common in the era, and scholars have long believed Fletcher was somehow involved—possibly assisting an aging Shakespeare.

Nineteenth-century literary analyst James Spedding theorized in 1850 that Fletcher had co-written the play; Fletcher had succeeded Shakespeare as the house playwright of the King’s Men Acting Company following the Bard's death in 1616. Spedding even surmised who wrote which scene. This most recent analysis loosely lines up with his findings.

[h/t Smithsonian]