As discussed in chapter 3, the actus reus of a crime is composed of: (1) a voluntary act (2) that causes (3) social harm. As all offenses contain an actus reus, causation is an implicit element of all crimes. Causation may be "actual" or "proximate."
[A] Common Law
 "But-for" test – There can be no criminal liability for resulting social harm "unless it can be shown that the defendant’s conduct was a cause-in-fact of the prohibited result." In order to make this determination, courts traditionally apply the "but-for" or "sine qua non" test: "But for the defendant’s voluntary act(s), would the social harm have occurred when it did?"
 Multiple Actual Causes – When a victim’s injuries or death are sustained from two different sources, any of the multiple wrongdoers can be found culpable if his act was "a" cause-in-fact of the injury or death. It is not necessary that any act be the sole and exclusive cause-in-fact of injury.
 Accelerating a result – Even if an outcome is inevitable – e.g., everyone dies – if defendant’s act accelerated death, he can be found criminally liable. The "but-for" test can be stated in such circumstances as "but for the voluntary act of the defendant, would the harm have occurred when it did?" E.g., a defendant shoots a terminally ill patient may still be found guilty of homicide since although the victim’s death was inevitable, it would not likely have occurred when it did but for the defendant’s unlawful act.
 Concurrent Causes – If, in the case of infliction of harm from two or more sources, each act alone was sufficient to cause the result that occurred when it did, the causes are concurrent and each wrongdoer can be found criminally liable.
 Obstructed Cause – If a defendant commits a voluntary act intending to cause harm – e.g., shooting a victim in the stomach intending to kill the victim – but another wrongdoer commits a more serious injury that kills the victim sooner, the initial wrongdoer might only be convicted of attempt to kill since the subsequent wrongdoer’s act obstructed his goal to killing the victim.
[B] Model Penal Code - The Model Penal Code applies the but-for (sine qua non) rule. [MPC § 2.03(1)(a).]
[A] Direct Cause – An act that is a direct cause of social harm is also a proximate cause of it.
[B] Intervening Causes – An "intervening cause" is an independent force that operates in producing social harm, but which only comes into play after the defendant’s voluntary act or omission; e.g., the intervention can occur as a result of wrongdoing by a third party, or as the result of a dangerous or suicidal act by the victim, or a natural force ("an act of God").
When an intervening cause contributes to the social harm, the court must decide whether such intervening cause relieves the defendant of liability. If so, the intervening event is deemed a "superseding cause" of the social harm.
 De Minimis Contribution to the Social Harm – In some cases, if the defendant’s voluntary act caused minor social harm compared to the social harm resulting from a substantial, intervening cause, the law will treat the latter as the proximate cause of the social harm.
 Foreseeability of the Intervening Cause – Some cases have held that the defendant cannot escape liability if the intervening act was reasonably foreseeable, whereas an unforeseeable intervening cause is superseding in nature. However, in determining foreseeability, the law tends to distinguish between "responsive" (or "dependent") and "coincidental" (or "independent") intervening causes.
A responsive intervening cause is an act that occurs as a result of the defendant’s prior wrongful conduct. Generally, a responsive intervening cause does not relieve the initial wrongdoer of criminal responsibility, unless the response was highly abnormal or bizarre. E.g., a defendant who wrongfully injures another is responsible for the ensuing death, notwithstanding subsequent negligent medical treatment that contributes to the victim’s death or accelerates it. However, grossly negligent or reckless medical care is sufficiently abnormal to supersede the initial wrongdoer’s causal responsibility.
A coincidental intervening cause is a force that does not occur in response to the initial wrongdoer’s conduct. The only relationship between the defendant’s conduct and the intervening cause is that the defendant placed the victim in a situation where the intervening cause could independently act upon him. The common law rule is that a coincidental intervening cause relieves the original wrongdoer of criminal responsibility, unless the intervention was foreseeable.
 Apparent Safety Doctrine – A defendant’s unlawful act that puts a victim in danger may be found to be the proximate cause of resulting harm, unless the victim has a route to safety but instead puts herself in further harm, which causes the injury of death. E.g., a spouse’s physical violence causes his wife to flee the house on a freezing night, and although the wife can find nearby shelter with a relative or friend, decides to spend the rest of the night outside, and dies from freezing temperature.
 Free, Deliberate, Informed Human Intervention – A defendant may be relieved of criminal responsibility if an intervening cause, e.g., a victim chose to stay outside in the freezing night and consequently died, was the result of a free, deliberate and informed human intervention. A subsequent dangerous action that caused the victim’s injury or death would not relieve the defendant of liability is such act resulted from duress.
 Omissions – An omission will rarely, if ever, serve as a superseding intervening cause, even if the omitter has a duty to act. E.g., a father’s failure to intervene to stop a stranger from beating his child will not ordinarily absolve the attacker for the ensuing homicide, although the father may also be responsible for the death on the basis of omission principles.
Unlike the common law, the "but-for" test is the exclusive meaning of "causation" under the Model Penal Code. The Code treats matters of "proximate causation" as issues relating instead to the defendant’s culpability. That is, in order to find the defendant is culpable, the social harm actually inflicted must not be "too remote or accidental in its occurrence from that which was designed, contemplated or risked. [MPC §2.03(2)(b), (3)(c)] In such circumstances, the issue in a Model Code jurisdiction is not whether, in light of the divergences, the defendant was a "proximate cause" of the resulting harm, but rather whether it may still be said that he caused the prohibited result with the level of culpability—purpose, knowledge, recklessness, or negligence—required by the definition of the offense.
In the rare circumstance of an offense containing no culpability requirement, the Code provides that causation "is not established unless the actual result is a probable consequence of the defendant’s conduct." [MPC § 2.03(4)] This would mean that in a jurisdiction that recognizes the felony-murder rule, but which applies Model Penal Code causation principles, a defendant may not be convicted of felony-murder if the death was not a probable consequence of his felonious conduct.