3.8: Substantive Law: Defenses

Kate McLean and Shanell Sanchez


Even if the government can prove all the elements of a crime, defendants may nevertheless raise defenses that result in their acquittal. Defense is a general term that includes perfect and imperfect defenses, affirmative and negative defenses, justifications and excuses, and procedural defenses.

Perfect and Imperfect Defenses

A perfect defense is one that completely exonerates the defendant. If the defendant is successful in raising this defense, meaning the jury believes him or her, the jury should find the defendant not guilty. An imperfect defense is one that reduces the defendant’s liability to that of a lesser crime. If the jury believes the defendant, it should find the defendant guilty of a lesser charge.

Negative and Affirmative Defenses

Sometimes the government is unable to prove all the elements of the crime charged. When this happens, the defendant may raise a negative defense claim. The defendant doesn’t have to prove anything; instead, they may simply argue that something is missing in the state’s case, that the state did not prove everything the statute said it had to prove, and therefore the jury should find them not guilty. For example, when charging a defendant with theft, the state must prove that the defendant intentionally took the property of another. If the jury finds that the defendant did not intend to take the property, or took property that was rightfully theirs, then it should find the defendant not guilty. Negative defenses, at their essence, are claims that there are “proof problems” with the state’s case. The defendant’s claim that the state failed to prove its case does not depend on whether the defendant has put out any evidence or not.

An affirmative defense requires the defendant to submit evidence that will persuade the jury that they should either be completely exonerated (for a perfect defense) or be convicted only of a lesser crime (for an imperfect defense). The defendant can meet this requirement by calling witnesses to testify, or by introducing physical evidence. Because of the presumption of innocence, the burden of proof  cannot switch completely to the defendant. The state must ultimately bear this burden, and prove the defendant’s guilt by putting out enough evidence that the defendant has committed the crime by proving each and every material element of the crime; it must convince the jury of this guilt beyond a reasonable doubt. However, when the defendant raises an affirmative defense, the burden of production or persuasion switches, at least in part and temporarily, to the defendant. The defendant’s burden is limited, however, to proving the elements of the defense he or she asserts.

Note the interplay of negative defenses and affirmative defenses. Even if a defendant is unsuccessful in raising an affirmative defense, the jury could nevertheless find him or her not guilty based upon the state’s failure to prove some other material element of the crime.


Sometimes doing the right thing results in harm. Society recognizes the utility of doing some acts in certain circumstances that unfortunately result in harm. In those situations, the defendant can raise a justification defense.  Justification defenses allow criminal acts to go unpunished because they preserve an important social value or because the resulting harm is outweighed by the benefit to society. For example, if a surgeon cuts someone with a knife to remove a cancerous growth, the act is a beneficial one even though it results in pain and a scar. In raising a justification defense, the defendant admits he did a wrongful act, such as taking someone’s life, but argues that the act was the right thing to do under the circumstances. At times, the state’s view differs from the defendant’s view of whether the act was, in fact, the right thing to do. In those cases, the state files charges, to which the defendant raises a justification defense.

Justification defenses include self-defense, defense of others, defense of property, defense of habitation, consent, and necessity, also called, choice of evils. Justifications are affirmative defenses for which the defendant must produce some evidence. In most cases, the defendant must also convince the jury through a preponderance of evidence that his or her conduct was justified. For example, the defendant may claim that they acted in self-defense, and at trial would need to call witnesses or introduce physical evidence that supports this claim. State law may vary with regards to how convinced the jury must be (called the standard of proof) or when the burden switches to the defendant to put out evidence, but all states generally require the defendant to carry at least some of the burden of proof in raising justification defenses.


Excuses are defenses to criminal behavior that focus on some characteristic of the defendant. With excuses, the defendant is essentially saying, “I did the crime, but I am not responsible because I was… (legally insane, too young, intoxicated, mistaken, or under duress).” Excuses include insanity, diminished capacity, automatism, age, involuntary intoxication, duress, and mistake of fact, among others. Like justifications, excuses are affirmative defenses in which the defendant bears the burden of putting on some evidence to convince the jury that he or she should not be held responsible for his or her conduct.


Look up one of the above “excuse” defenses, and briefly summarize its requirements. Then, see if you can find a recent or famous case that utilized your chosen “excuse” defense.

Procedural Defenses

Procedural defenses are challenges to the state’s ability to bring the case against the defendant for some reason. These defenses point to some problem in the prosecution process, or the state’s lack of authority to bring the case. Procedural defenses include: double jeopardy (a defense in which the defendant claims that the government is repeatedly and impermissibly prosecuting him or her for the same crime), speedy trial (a defense in which the defendant claims the government took too long to get his or her case to trial), entrapment (a defense in which the defendant claims the government in some way enticed them into committing the crime), the statute of limitations (a defense in which the defendant claims the government did not charge them within the required statutory period), and several types of immunity (a defense in which the defendant claims they are immune from being prosecuted). Although procedural defenses are considered procedural criminal law, many states include the availability of these defenses in their substantive criminal codes.

Watch: When Is Self-Defense Justified?

In 2013, Michigan resident Theodore Wafer was charged with the murder of Renisha McBride, a 19-year-old woman who had crashed her car near Wafer’s house. Hearing McBride banging on his door at 4:42 AM, Wafer fired a shotgun through the door, fatally wounding the young woman. Watch Wafer testify at trial below. How would you vote as a member of the jury?


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Introduction to the U.S. Criminal Justice System Copyright © 2019 by Kate McLean and Shanell Sanchez is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License, except where otherwise noted.

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