Mens rea is a Latin term meaning guilty mind. To be convicted of a crime, the law requires the defendant to be deserving of punishment based on culpability or blameworthiness.
Take the case of the killing of Trayvon Martin. A jury believed George Zimmerman's story that he shot Trayvon Martin in self-defense. There was never any dispute that Zimmerman shot Martin. The dispute at trial turned on what was in Zimmerman's mind when he fired the gun. In order to convict Zimmerman, the prosecution had to prove that Zimmerman had criminal intent—mens rea. It failed to do that and Zimmerman was acquitted.
In the American legal system, every crime has two components which the prosecution must prove to obtain a conviction of the defendant:
If either component is missing, the defendant will be acquitted.
For example, a woman who repeatedly tells her friends that she wishes her husband was dead cannot be convicted of murder based on her wish alone if the husband goes missing. In that situation, the actus reus is missing (literally). Or say a woman who slips (really) in the kitchen while deboning a chicken and accidentally eviscerates her husband is not guilty of murder either, because she lacked the mens rea for murder (she did not intend to kill him).
But criminal intent does not necessarily mean malicious intent, and a person can be found to have criminal intent even where his motives are objectively good. The required intent is the intent to commit the prohibited act. For example, an Earth First activist who breaks into a logging company's yard in order to prevent disable logging machinery might have good intentions (saving endangered old-growth forests), but if the prosecution proves that he intended to forcibly enter the yard without authorization and destroy the property of others, he will be found guilty. The criminal intent for the trespass was simply the activist's intentional entry onto the private property of another.
Different levels of criminal intent are required to prove different crimes. The levels range from no intent (strict liability, which is rare in criminal cases) to the level of intent required to obtain a death penalty verdict in a first-degree murder trial. Generally speaking, low-level crimes that carry misdemeanor penalties or fines only typically require proof of lower levels of criminal intent. The greater the crime and possible sentence, the higher the level of criminal intent that must be proven.
The highest degree of criminal intent is malice aforethought, which is usually required to prove first-degree murder. A person acts with malice aforethought when she acts with prior intent to do the criminal act. Where a person intends to kill another prior to taking that action and then does kill the other person, the killer has acted with malice aforethought.
When a person intends to do the criminal act but does not form the intent prior to taking the action (which would constitute malice aforethought), the person has acted with intent. So, a person who gets into an argument with another person and pulls a gun and shoots the other person has acted with the intent to kill the victim. But, the killer did not form the intent to kill prior to the killing.
Many states have voluntary manslaughter laws. Voluntary manslaughter is usually defined as the intentional killing of a person but in the heat of passion or some other provocation. A "cold-blooded" intentional killing would generally be charged as a murder.
A person who acts without a specific intent to do a criminal act but whose intentional actions are such that the person knew or should have known that the wrongful result would occur has acted knowingly under the law. Where a person fires a gun into a crowd, the person might not intend to kill any particular person but acted in a manner knowing that the death of a victim was a likely result of the shooter's action.
Some states, including California, require only proof that the defendant had knowledge that death was a likely result of her actions to support a second-degree murder conviction.
A person acts with reckless disregard when he consciously ignores a "substantial and unjustifiable risk" that his actions will result in a crime or harm to another. Anyone who engages in reckless conduct may also be found guilty of any crime that results from that conduct. Vehicular homicide laws often require this level of mens rea—so a person who drives recklessly and kills a pedestrian could be convicted of vehicular homicide.
Involuntary manslaughter typically includes a killing done by someone acting with reckless disregard for the consequences of their actions.
Virtually all crimes listed in state and federal criminal codes require some degree of mens rea. However, lawmakers have decided that some offenses require consequences even if a person did not intend to commit the prohibited act. These crimes are strict liability crimes.
A common example of a strict liability offense is a traffic violation, such as speeding. Anyone who's ever tried to convince a cop that she didn't realize she was speeding knows that the offender's mental state is completely irrelevant. The "actus reus" (speeding) is enough by itself to justify a citation.
Strict liability offenses tend to carry lower-level penalties (such as a citation or misdemeanor penalty). An exception being, that in many states, "statutory rape"— consensual sex with an underage person—remains a strict liability crime that can carry felony penalties.
As mentioned, the greater the severity of the crime and the possible penalty for committing it, the greater the level of mens rea that the state must usually prove to convict the person charged.
A defendant who lacks the mental capacity to form criminal intent cannot be convicted under most state laws. The "insanity" defense falls within this definition.
Minors younger than 18 may also be considered to lack the capacity to form requisite criminal intent simply based on their youth under some state laws.
The mens rea element of criminal law generally eliminates the excuse that a defendant was ignorant of the law and so should be acquitted. In the United States' legal system, a person is liable for the legal effects of intended acts. But, if "willfulness" is an element of a crime, meaning that the defendant knowingly and intentionally violated a law, ignorance of the law would be a defense to that element.
Some defenses require acquittal even where the state has proven actus reus and mens rea. These defenses include self-defense and justifiable homicide. George Zimmerman's attorneys argued to the jury that Zimmerman acted in self-defense and offered evidence that they say showed that Trayvon Martin was attacking Zimmerman when Zimmerman shot him. This evidence persuaded the jury that Zimmerman acted in self-defense and the jury acquitted him despite the fact that he intended to, and did, kill Martin.
Given the many shades of criminal intent that the American legal system recognizes, the frequent jail-house refrain, "I'm innocent," turns into a loaded phrase. The way intent is viewed under the law is complex and specific to each crime. If you have questions about criminal intent in particular crimes, talk to an experienced criminal defense lawyer in your area.