More than 100 years after Shakespeare’s death, Lewis Theobald published Double Falsehood, a play supposedly sourced from a lost play by Shakespeare and John Fletcher. Since its release, scholars have attempted to determine its true authorship. Using new approaches to language and psychological analysis, we examined Double Falsehood and the works of Theobald, Shakespeare, and Fletcher. Specifically, we created a psychological signature from each author’s language and statistically compared the features of each signature with those of Double Falsehood’s signature. Multiple analytic approaches converged in suggesting that Double Falsehood’s psychological style and content architecture predominantly resemble those of Shakespeare, showing some similarity with Fletcher’s signature and only traces of Theobald’s. Closer inspection revealed that Shakespeare’s influence is most apparent early in the play, whereas Fletcher’s is most apparent in later acts. Double Falsehood has a psychological signature consistent with that expected to be present in the long-lost play The History of Cardenio, cowritten by Shakespeare and Fletcher.