Black-letter propositions
- 01
A voluntary movement or knowingly retained possession can satisfy the act requirement; status alone cannot.
- 02
An omission is punishable only when a legal duty, knowledge, and ability to act exist.
- 03
Model Penal Code culpability descends from purposely to knowingly, recklessly, and negligently; common-law offenses use their stated mental states.
- 04
Transferred intent can move an intended harm to an unintended victim for qualifying offenses.
- 05
Result crimes require actual and proximate causation; an unforeseeable superseding intervention can break the chain.
Use this attack sequence under time
Frame the issue
Match a mental state to every material element.
Resolve the next checkpoint
Find a voluntary act or legal duty to act.
Resolve the next checkpoint
Confirm concurrence.
Finish the conclusion
For result crimes, prove actual and proximate cause.
Common wrong turns
- Strict liability must be clearly indicated.
- Ordinary medical negligence usually does not supersede a violent actor's causation.
- A factual mistake matters only if it negates the required mental state.
Worked example
A parent able to call for help knowingly leaves a dependent infant in medical distress, and the infant dies.
A parent has a legal duty to protect the child, so the omission can be the actus reus. The prosecution must still prove the required culpability and that timely aid would have prevented death.
Practice this topic
The act requirement
Cornell defines actus reus as the act or omission that comprises the physical elements of a crime as required by statute. It includes only a voluntary affirmative act, or an omission causing a criminally proscribed result, and an involuntary act, such as a reflex, does not satisfy the requirement. An omission counts only when the person had a duty to act. Cornell lists sources of that duty: a statute, a contract, a special status relationship such as parental responsibility, a voluntary assumption of care, and creating the peril.
Mens rea is the separate mental element, the state of mind statutorily required for the crime. Cornell says courts usually apply a stated mental state to each element of the offense, so match act, mental state and result element by element.
Causation
Cornell describes a proximate cause as an actual cause that is also legally sufficient to support liability. It says the likelihood of calling something a proximate cause increases as the cause becomes more direct and more necessary for the injury, and that the term is used for tort and criminal liability. It gives two common tests: foreseeability of the harm as a consequence of the action, and the substantial factor test.
In a homicide or other result crime, prove actual cause first, then ask whether the result is legally attributable to the defendant.
Frequently asked questions
What is the controlling rule for act, culpability, and causation?+
Criminal liability requires a voluntary act or duty-based omission accompanied by the prescribed mental state and, for a result crime, legal causation.
What is the actus reus requirement?+
The act or omission comprising the physical elements of the crime, limited to a voluntary act or an omission by a person with a duty to act.
Does an involuntary act satisfy the act requirement?+
No. Cornell says a reflex or other involuntary movement does not satisfy the actus reus requirement.
When is an omission enough for criminal liability?+
When the person had a duty to act, which can arise from a statute, a contract, a special relationship, a voluntary assumption of care or creating the peril.
What is proximate cause?+
An actual cause that is also legally sufficient to support liability, and its likelihood rises as the cause becomes more direct and more necessary, according to Cornell.
What tests are used for proximate cause?+
Foreseeability of the harm as a consequence of the act, and the substantial factor test, per Cornell's summary.