Actus Reus, Mens Rea, Concurrence, and Causation
Key Takeaways
- Every crime requires an actus reus (voluntary act or qualifying omission) plus, for most crimes, a culpable mens rea; an involuntary act (reflex, seizure, unconsciousness, sleepwalking) cannot supply actus reus.
- Common law recognized four mental states (specific intent, malice, general intent, strict liability); the MPC reduced these to four hierarchical levels: purposely, knowingly, recklessly, and negligently.
- Concurrence requires that the mens rea actuate the actus reus — the guilty mind must exist at the moment of the act and set the criminal conduct in motion.
- Causation has two parts: actual cause ('but for' the act the result would not have occurred) and proximate (legal) cause, which fails only when an intervening cause is unforeseeable and superseding.
- Transferred intent applies to completed harms (e.g., D shoots at A, hits B — intent transfers), and supports attempt as to the intended victim; it does not transfer across different crimes.
Criminal liability begins with a voluntary act — a willed bodily movement. The law does not punish thoughts, status, or conditions; Robinson v. California held that punishing a person merely for the status of being a narcotics addict violates the Eighth Amendment. Conduct that is not the product of the actor's volition is not an actus reus: reflexes, convulsions, movements during unconsciousness or sleep, and conduct under hypnosis are all involuntary.
A common MBE trap is the epileptic driver — if a defendant has a seizure at the wheel and crashes, the seizure itself is involuntary, but liability can attach to the earlier voluntary act of choosing to drive knowing seizures were likely. Omissions create liability only where there is a legal duty to act and the defendant can physically perform it. Duties arise from five sources: (1) statute (e.g., filing a tax return), (2) contract (a lifeguard or nurse), (3) special relationship (parent-child, spouse), (4) voluntary assumption of care that isolates the victim from other help, and (5) creation of the peril.
A bystander who watches a stranger drown owes no duty; a parent who watches their own child drown commits criminal homicide by omission. Possession also counts as an act where the defendant knowingly procured or received the thing and had a chance to dispose of it.
The bar tests two parallel vocabularies. At common law, crimes fall into four mental-state buckets. Specific-intent crimes require proof that the defendant intended a particular result beyond the act itself, and they unlock special defenses (voluntary intoxication and unreasonable mistake of fact). The classic specific-intent crimes are first-degree premeditated murder, assault (attempted battery), larceny, embezzlement, false pretenses, robbery, burglary, forgery, conspiracy, attempt, and solicitation — a frequently memorized list. Malice crimes (common-law murder and arson) require a reckless disregard of a high risk of harm.
General-intent crimes (battery, rape, kidnapping, false imprisonment) require only an intent to do the proscribed act. Strict-liability crimes (statutory rape, regulatory/public-welfare offenses) require no mens rea at all and bar mistake-of-fact defenses. The Model Penal Code abolishes this taxonomy and substitutes four culpability levels applied element-by-element: purposely (conscious object to cause the result), knowingly (aware the result is practically certain), recklessly (conscious disregard of a substantial and unjustifiable risk — the MPC default when a statute is silent), and negligently (should have been aware of the risk).
Recklessness is subjective; negligence is objective.
Concurrence demands that the mens rea and the actus reus exist at the same moment, and that the intent actuate the act. If D breaks into a house intending only to escape a storm, sees jewelry, and then decides to steal, there is no burglary (the intent to commit a felony did not exist at the time of breaking and entering). Causation requires both actual cause and proximate cause. Actual cause is the 'but for' test: but for D's conduct, would the result have occurred when it did? Where two defendants independently inflict simultaneous fatal wounds, courts use the 'substantial factor' test instead.
Proximate cause asks whether the result is a foreseeable consequence; an intervening cause cuts off liability only if it is both unforeseeable and superseding. Negligent medical treatment of a wound the defendant inflicted is foreseeable and does not break the chain; grossly negligent or intentional malpractice may. Acts of nature and acts of the victim (e.g., refusing treatment) are usually foreseeable. The 'eggshell victim' rule means a defendant takes the victim as found — that the victim was unusually frail does not break causation.
Transferred intent completes the structural picture. When a defendant intends to harm one victim but, by bad aim or mistake, harms a different victim, the law 'transfers' the intent from the intended to the actual victim so the completed crime is treated as intended. D shoots at A, misses, and kills B: D is guilty of murdering B (transferred intent) and may also be guilty of attempted murder of A (the original intent supports the attempt).
The doctrine operates only within the same crime and only for the same general type of harm — intent to commit one crime does not transfer to a different, unrelated crime, and it does not apply where the resulting harm is wholly different in kind from the intended harm. A second high-yield refinement is element-by-element mens rea under the MPC: a single offense can require different culpability levels for different elements.
Where a statute prescribes a mens rea but is ambiguous about which elements it governs, the MPC presumes the stated level applies to every material element; where the statute is silent altogether, recklessness is the default. Compare the common law, which tends to attach a single label (specific intent, malice, general intent) to the whole crime. This produces the classic exam trap distinguishing knowledge from purpose: a defendant who plants a bomb to kill one passenger but is practically certain the explosion will kill others acts purposely as to the target and knowingly as to the others — both satisfy murder's malice.
Three further traps recur. First, 'willful blindness' (deliberately avoiding confirmation of a fact) usually satisfies knowledge. Second, mistake of fact interacts with mens rea: an honest mistake negates specific intent even if unreasonable, but only a reasonable mistake negates general intent, and no mistake aids strict liability — a point that ties causation and concurrence back to the mental element. Third, do not confuse motive with intent; a good motive (mercy, financial desperation) does not negate the intent element and is generally irrelevant to guilt, bearing only on sentencing.
California codifies the act/intent union in Penal Code section 20, which provides that in every crime there must exist a union, or joint operation, of act and intent or criminal negligence — a direct statutory statement of the concurrence principle. Penal Code section 26 separately excuses persons who act without volition (e.g., through unconsciousness), reinforcing that an involuntary act is no actus reus. Keep these threads distinct on an essay: identify the act, fix the required mental state element by element, confirm the two coincided in time, and only then trace actual and proximate causation to the result.
Mens Rea Crosswalk: Common Law vs. MPC
| Common-Law Category | Example Crimes | Closest MPC Level | Defenses Unlocked |
|---|---|---|---|
| Specific intent | First-degree murder, larceny, burglary, attempt, conspiracy | Purposely / Knowingly | Voluntary intoxication; unreasonable mistake of fact |
| Malice | Common-law murder, arson | Recklessly (high degree) | No intoxication/mistake defense |
| General intent | Battery, rape, kidnapping, false imprisonment | Recklessly / Knowingly | Only reasonable mistake of fact |
| Strict liability | Statutory rape, public-welfare offenses | None required | No mistake-of-fact defense |
Dan, who has a documented history of epileptic seizures, chooses to drive to work despite his doctor's warning. He suffers a seizure at the wheel, loses consciousness, and his car kills a pedestrian. Which statement best describes Dan's potential criminal liability?
Vince stabs Walt, inflicting a wound that would normally be survivable. Walt is taken to the hospital, where a physician commits ordinary negligence in suturing the wound, and Walt dies of an infection. On these facts, what is the most likely causation result?