OK, Shakespeare stole. We know that. He stole repeatedly, sometimes a pick-pocketing, sometimes grand larceny. But what about for Cymbeline?
The question is clear; the answer somewhat muddled.
It seems that Shakespeare dipped into his usual historical well of tricks for his title and the setting: Holinshed’s Chronicles. Only here, it seems that Holinshed had done a little larceny himself, using the history of Britain’s ancient kings, Historia Regum Britanniae by Geoffrey of Monmouth, written around 1136. In this work, king Kymbelinus and his sons Guiderius and Arvirargus are mentioned. According to both writers, these men lived in the times of the Roman conquest of England. And that’s about all Shakespeare takes from history (but more on that in a later entry).
For Cymbeline, Shakespeare also stole from another of the usual suspects: Boccaccio’s Decameron. This is where Shakespeare finds the story of the bet over a woman’s virtue. By Shakespeare’s time, this story had become a kind of stock plot. The Decameron also contains an analog to the trunk-hiding scheme by Iachimo. And, of course, there are call-outs to Ovid’s Metamorphoses.
It’s conceivable that Shakespeare lifted some elements, sub-plots (like Posthumus’ banishment and Cloten’s jealousy) from The Rare Triumphs of Love and Fortune, written in 1589 probably by Thomas Kyd.
So much for old thievery…
It’s also possible that Shakespeare was influenced by two contemporary plays by Francis Beaumont and John Fletcher (the latter’s hands are all over Cardenio, Henry VIII, and The Two Noble Kinsmen), Philaster and Cupid’s Revenge, both of which contain plots about a disobedient princess who marries below her station, subsequent claims of adultery, and a character named Belarius. Another play written around the same time, The Golden Age by Thomas Heywood, has the theatrical act of Iris descending and ascending; it’s not clear if Shakespeare’s use of Jupiter in this play influenced Heywood or vice-versa.
It seems, however, that the whole “stolen sons” subplot (and thus much of the end of the play) is Shakespeare’s original work.