In February 1989 he was asked by the Shakespeare Authorship Roundtable to investigate the scholarship of the Shakespeare authorship controversy and was invited to speak to the membership in Los Angeles about his findings.
Matus, according to Jonathan Bate, came to the controversy as an agnostic.
 Up until this particular research interest emerged, his interest in Shakespeare had been almost entirely focused on the plays, not their author.
He flew to Los Angeles in August 1989 and spent two months there.
His visit culminated in an address he delivered to members of the Roundtable in September, in which he remarked that, though he found the members honest, thoughtful, gracious, and cordial, his research found much fault with Oxfordian scholarship and led him to the conclusion that William Shakespeare was the true author of the works.

Matus then turned to write a comprehensive book on the Shakespearean authorship question.
He returned to England in 1993 for another five months, primarily to research in the British Library.
In Matus's view, the long history of the debate in public controversy and both amateurish and professional speculation had been usually ignored by scholars.
He defended his method by arguing for the following principle:
It is the rule of controversialist scholarship, the error rate of which hovers around 100 percent, that a single flaw in a work of orthodox scholarship, whether perceived or actual—or fabricated—is sufficient in their eyes to cast doubt upon the accuracy and authenticity of the entire work.

He took the "Stratfordian" perspective, advocating the orthodox view that William Shakespeare of Stratford-on-Avon, was the primary author of the plays attributed to him.
 Matus went on to defend this position against the Oxfordian theory, in the October 1991 issue of The Atlantic Monthly as part of a print debate written by advocates of both sides.
He did the same in an article published as part of a similar debate in the April 1999 issue of Harper's Magazine.

In Shakespeare, In Fact, Matus examined Oxfordian arguments, presenting both detailed rebuttals of the sceptical perspective and positive evidence for Shakespeare's authorship.
On its publication, Shakespeare scholar Thomas Pendleton hailed the work as the most authoritative on the subject.
Writer Scott McCrea has praised it for its "original and valuable scholarship",  while James S. Shapiro has recently referred "those interested in the strongest arguments in favor of Shakespeare’s authorship" to Matus's book.
Gail Kern Paster, director of the Washington, DC-based Folger Shakespeare Library, judging its contribution more broadly, has written that Shakespeare, in Fact is recognized as "a reliable, trustworthy, and authoritative source for what we know for sure about Shakespeare."
Dover Books reissued  Shakespeare, In Fact in 2013, with a new introduction by Thomas Mann, former reference librarian at the Library of Congress.

While working on Shakespeare, In Fact Matus also investigated a different authorship question: Charles Hamilton's claim that he had found "Cardenio"—a lost play by Shakespeare.
The play turned out to be the playhouse copy of Thomas Middleton's "The Second Maiden's Tragedy," and Matus reviewed Hamilton's book for the Times Literary Supplement.
In February 1989 he was asked by the Shakespeare Authorship Roundtable to investigate the scholarship of the Shakespeare authorship controversy and was invited to speak to the membership in Los Angeles about his findings. Matus, according to Jonathan Bate, came to the controversy as an agnostic.  Up until this particular research interest emerged, his interest in Shakespeare had been almost entirely focused on the plays, not their author. He flew to Los Angeles in August 1989 and spent two months there. His visit culminated in an address he delivered to members of the Roundtable in September, in which he remarked that, though he found the members honest, thoughtful, gracious, and cordial, his research found much fault with Oxfordian scholarship and led him to the conclusion that William Shakespeare was the true author of the works.
Matus then turned to write a comprehensive book on the Shakespearean authorship question. He returned to England in 1993 for another five months, primarily to research in the British Library. In Matus's view, the long history of the debate in public controversy and both amateurish and professional speculation had been usually ignored by scholars. He defended his method by arguing for the following principle:
It is the rule of controversialist scholarship, the error rate of which hovers around 100 percent, that a single flaw in a work of orthodox scholarship, whether perceived or actual—or fabricated—is sufficient in their eyes to cast doubt upon the accuracy and authenticity of the entire work.
He took the "Stratfordian" perspective, advocating the orthodox view that William Shakespeare of Stratford-on-Avon, was the primary author of the plays attributed to him.  Matus went on to defend this position against the Oxfordian theory, in the October 1991 issue of The Atlantic Monthly as part of a print debate written by advocates of both sides. He did the same in an article published as part of a similar debate in the April 1999 issue of Harper's Magazine.
In Shakespeare, In Fact, Matus examined Oxfordian arguments, presenting both detailed rebuttals of the sceptical perspective and positive evidence for Shakespeare's authorship. On its publication, Shakespeare scholar Thomas Pendleton hailed the work as the most authoritative on the subject. Writer Scott McCrea has praised it for its "original and valuable scholarship",  while James S. Shapiro has recently referred "those interested in the strongest arguments in favor of Shakespeare’s authorship" to Matus's book. Gail Kern Paster, director of the Washington, DC-based Folger Shakespeare Library, judging its contribution more broadly, has written that Shakespeare, in Fact is recognized as "a reliable, trustworthy, and authoritative source for what we know for sure about Shakespeare." Dover Books reissued  Shakespeare, In Fact in 2013, with a new introduction by Thomas Mann, former reference librarian at the Library of Congress.
While working on Shakespeare, In Fact Matus also investigated a different authorship question: Charles Hamilton's claim that he had found "Cardenio"—a lost play by Shakespeare. The play turned out to be the playhouse copy of Thomas Middleton's "The Second Maiden's Tragedy," and Matus reviewed Hamilton's book for the Times Literary Supplement.
Irvin Leigh Matus