Shakespeare used sources for almost all of his plays. By studying these sources, we can gain a greater understanding of the play, and aid our interpretation of it. One of Shakespeare's favorite sources of inspiration was Raphael Holinshed's Chronicles of England, Scotland and Ireland. Written in 1587, a few years before Shakespeare began his career, Holinshed's Chronicles served as the primary source of his Histories, King Lear, and the topic of our source study, Macbeth. Reading the account of MacBeth in the Chronicles feels almost like reading an imperfect "Cliff's Notes" version of the play: it very briefly covers the same plot points, but many of the details are changed. Shakespeare deviates from his source in for many reasons, political and dramatic.

The single most important idea to consider when evaluating any source study, or indeed, any work of fiction or non-fiction is to take into account the intended audience. When Macbeth was written, Shakespeare's acting company had recently come under the patronage of King James I, and the play was written primarily with the new King in mind. Shakespeare tailored Macbeth to the interests and tastes of King James. The Norton Shakespeare's introduction says that:

Consequently, Shakespeare expanded the role of the Weird Sisters in MacBeth's rise and fall. The source mentions the Weird Sisters as giving their prophecy to MacBeth and Banquo, but the source says that MacBeth got his further prophecies from witches which he employed and consulted. Shakespeare most likely made this change because he understood the important role the Weird Sisters play in the drama as agents of the devil to whom MacBeth willingly returns.

The King's interest in the occult also probably led to the scenes featuring the "spectral dagger" and the Ghost of Banquo, visible only to MacBeth, although the inclusion of the infamous Hecate is most likely a later addition by a later play doctor.

A major reason why Shakespeare chose to dramatize the story of Macbeth may have been in order to please the king, as James I traced his ancestry to Banquo and Fleance. This reason, however, led to major changes from the source. In Holinshed's Chronicles, Banquo was a willing co-conspirator of MacBeth's in the murder of King Duncan. Shakespeare chooses to make Banquo more of a hero and victim. Although Banquo hears the same prophecies as MacBeth from the Weird Sisters, but when they begin to come true, he doesn't actively seek to bring them about, as MacBeth does, and doesn't conspire against King Duncan. He doesn't fall into evil as MacBeth does, and in the play, the murder is committed by a conspiracy of two: MacBeth and his wife.

James' belief that a plot against an anointed king was comparable to heresy led to several changes as well. Holinshed's Chronicles records that King Duncan was a mediocre king at best, while Shakespeare chose to idealize King Duncan, in order to contrast to MacBeth and his crimes. According to Holinshed's Chronicles, the historical MacBeth reigned as a very strong and competent king for ten years before paranoia and fear caused him to turn against his friends and fellow nobles, and become a tyrant. Shakespeare further demonizes MacBeth by causing him to make these mistakes immediately, emphasizing the rightness of MacDuff and Malcolm in removing him from the throne.

No matter who Shakespeare's intended audience was, he was first and foremost a dramatist. Many of the structural changes to Macbeth were toward this end. Historically, Macbeth's reign lasted seventeen years, although Shakespeare intensified it by shortening the time span of the play to a matter of months. This makes the play very intense, and helps the audience to have some understanding and perhaps sympathy for MacBeth's situation. His irrationality and paranoia are partially due to misinformation and being forced into action. Holinshed does record that Lady MacBeth, "greatly encouraged him hereunto" (Holinshed, 142), and Shakespeare's Lady MacBeth actively berates MacBeth into killing Duncan, going so far as to implicate the guards herself, bloodying her own hands as well. MacBeth's moral degradation is certainly also influenced by the Weird Sisters, who give the advice that he should "Beware MacDuff", as well as the other, more misleading prophecies. The first prophecies of the Weird Sisters also lead to MacBeth's mistrust and betrayal of Banquo, as MacBeth envies Banquo his heirs, who are destined to be a great line of kings. Shakespeare brings the plight of the criminal MacBeth to a very human level, and even though the audience can't condone his actions, they can understand his the human motives that led to the bloody realization of his murderous ambition.

This sympathy is the way in which Shakespeare was able to turn a historical narrative, almost belonging in the category of Shakespeare's Histories, into a profound work of tragedy. This human core is in the heart of all of Shakespeare's greatest works, as he can chronicle the rise and fall of kings and kingdoms in the terms of feeling and thinking humans.

This human core was also present in the way that Shakespeare deviated from his source to include the fourth-act pathos of the murder of MacDuff's family. According to Holinshed, MacBeth led an army to the castle of MacDuff, with the intention of killing him. The castle falls without a fight as the doors are opened to the king, and MacDuff's entire family is slaughtered. Shakespeare's MacBeth sends assassins to murder MacDuff and his family, which further illustrates MacBeth's decent, as he now sends paid murderers to eliminate his enemies, but also creates a very Shakespearean example of fourth-act pathos.

Shakespeare's genius lies in the sympathy and understanding we have for his tragic heroes. More important to understanding Shakespeare than the individual deviations are the reasons behind them. The study of Shakespeare's sources allows us to better understand his creative processes and agendas, and this understanding leads to a greater appreciation for the immortal bard and his works.