From "Come you spirits that tend on mortal thoughts" to "no man of woman born," you'd think all the curses you'd need would be in the play, right? In fact, the tragedy's surprisingly ambivalent relationship with the supernatural ends up being the key to the need for an outside mystical intervention.

The most entertaining account of the Macbeth curse, in my opinion, is in Paul Menzer's Anecdotal Shakespeare: A New Performance History. The basic premise of the book is right there in its titles: anecdotes. Theatre people love stories, and love telling stories, and love telling stories about theatre people. The gossipy, close-knit world of performers and crew that swirls around a core of often-performed plays like Shakespeare's became results in almost a canon of second- and thirdhand anecdotes: stories that you heard from this person who heard it from this person. As Menzer traces, these anecdotes are literary productions of their own, with the same story being told about different actors in different productions in different years--or centuries.

For example, John Manningham described in 1602 how Richard Burbage, playing Richard III, and William Shakespeare had taken a fancy to the same lady in the audience. Taken by the performance, she invited Burbage to her home that night--but Shakespeare heard of the invitation and beat him to it: "William the Conqueror was before Richard the Third." Fast forward nearly four centuries, and Richard Harris has a parallel story to tell arising from his friendship with fellow actor Peter O'Toole. They, too, fancied the same woman, and decided that whoever got there first would have first dibs at wooing. O'Toole won: "Peter the Great comes before Richard, etc."

In this world of stories/rumours/things that definitely happened to this person you know--that is to say, the very format of ghost stories--surely we can trace a long history of bad things that happened or "happened" during productions of Macbeth. And indeed, we can.For example, an audience member in 1671 wrote:

It is reported that Harris has killed his associate actor, in a scene on the stage, by accident. I twas the tragedy called 'Macbeth', in which Harris performed the part of Macduff, and ought to have slain his fellow-actor Macbeth; during the fence it happened that Macduff pierced Macbeth in the eye, by which thrust he fell lifeless, and could not bring out the last words of his part.

Nor do we hear only of accidents onstage:

At the Acting of this Tragedy, on the Stage, I saw a real one acted in the Pit; I mean the Death of Mr. Scroop, who received his death's wound from the late Sir Thomas Armstrong, and died presently after he was remov'd to a House opposite to the Theatre. (1690)

Nor do we hear of misfortune only on stage:

One day [star of 1916 short film Macbeth] Herbert Beerbohm-Tree stole away from the studio to keep a luncheon engagement. As he left his host's bungalow he slipped and fell against the mudguard of his motor. When he was lifted up blood was streaming from a ragged cut under his eye. A physician quickly patched him up; he worked all the afternoon and evening with makeup concealing the wound.

Oh, yeah, and the actor who played Macduff broke his wrist and had to keep filming the swordfighting scenes.

But as Menzer observes, whatever the reality of "it is reported that," there is a common feature to Macbeth anecdotes well into the 20th century. Despite any number of disasters, none of them invoke any kind of curse. In fact, the cursed play of centuries' worth of anecdotes was generally (really) All's Well that Ends Well.

In 1937, however, the stars crossed against a particularly ill-fated--and equally to the point, ill-reviewed--production of the play at the Old Vic in London. The theatre manager, Lilian Baylis, died; the director was in a massive car accident; star Laurence Olivier lost his voice; the set was designed wrong and had to be redone at the last minute; Olivier nearly died when a giant stage weight crashed down where he had been sitting just moments before; and to top it all off Baylis' poor dog died. Well, if you've got all that misfortune and you still have to market a production getting horrible reviews, which stories--which ancedotes--are you going to put into wider circulation?

And while All's Well that Ends Well lends itself to the accumulation of "curse" anecdotes for anyone who loves a good bit of irony, specific qualities of how the Scottish play handles the supernatural made it a prime target in the 1900s-30s, when Menzer and others figure the idea of a curse was building quietly. (Scholarly editions of the play generally comment that the curse was "well known by 1937", or some such).

The story of Macbeth is eminently steeped in superstition and magic on one hand (Double, double, toil and trouble)--and on the other, surprisingly resistant to it ("No man of woman born" is not supernatural but rather refers to a Caesaran section). The early 20th century was absolutely obsessed with the supernatural as well, but obsessed with rationalizing it and making it scientific. As Menzer puts it, "the curse restores what the play disowns"--and began to do so exactly when it is needed to serve the same role in contemporary culture. At the end of the 19th century, reviewers had started to complain that the witches and supernatural melodrama spoiled what was otherwise a "sublime" play, because no enlightened modern playgoer could buy into such a premise. And so anecdotes of bad luck coalesce into evidence for a curse, restoring the superstitious into the properly supernatural.

While /u/sunagainstgold's reply delves neatly into the general aspects of how anecdotes and stories become theatrical tradition, and from this evolve into a curse, it misses the largest single event that became the central thrust behind the Macbeth curse - the Astor Place Riot.

Two of the most famous actors of the 19th century, the British William Macready and the American Edwin Forrest, had a longstanding rivalry that stemmed from their cultural divergence in approaching the Bard. Forrest took a proto-naturalistic approach to performance that went against the English traditional sensibility of Macready, and Macready's frequent presence in New York rubbed Forrest the wrong way, as he was stepping into American territory with his very English dramatic interpretation.

The Astor Place Riot occurred when Macready brought a performance of Macbeth to the Astor Place, a major New York theatre, and Forrest opened his own production at the Bowery. The two major actors - and their fans - spurred a feud in the city that culminated in Forrest's supporters launching an assault on the Astor Place during a performance while police and soldiers fired into the mob, ending with more than a score dead and many injured.

Supposedly (according to theatrical tradition and the play Two Shakespearean Actors) this incident is the cause of saying "the Scottish play" or "Mackers" rather than Macbeth when inside a theatre, to ward against the curse. Because of the shared British and American memory of this incident, it's a transatlantic tradition.

Though there are many other instances of the curse's supposed effects both before and after the Astor Place Riot, it still stands as the most deadly theatrical event in American history - which, considering a presidential assassination during a performance of Our American Cousin a little over a decade later, is a surprisingly notable feat.