General principles - Acts and omissions

Learning Outcomes

This article examines the foundational criminal law principle of actus reus in an MBE context, including:

• Clarifying the requirement of a voluntary physical act and distinguishing it from involuntary movements such as reflexes, convulsions, or acts during unconsciousness.
• Showing how courts "time‑frame" conduct so that earlier voluntary behavior can supply the necessary actus reus even when the immediate harm results from an involuntary movement.
• Identifying when a failure to act, combined with a legally recognized duty and the capacity to perform that duty, satisfies the actus reus element.
• Comparing the main sources of legal duties—statute, contract, special relationship, voluntary assumption of care with seclusion, and creation of risk—and how each appears in multiple‑choice fact patterns.
• Explaining the additional requirements for omission liability, including awareness of the triggering circumstances, physical ability to act, and causal contribution to the result.
• Detailing how possession (actual and constructive) operates as a criminal act, with emphasis on control, awareness, and sufficient duration of possession.
• Differentiating knowing possession from mere presence near contraband and recognizing when joint or constructive possession is likely on the exam.
• Training you to spot when the actus reus element is missing—because the act is involuntary, no legal duty exists, or possession is not sufficiently established—so that you can eliminate tempting but incorrect answer choices.

MBE Syllabus

For the MBE, you are required to understand the essential elements of a crime, particularly the requirement of a physical act (actus reus), with a focus on the following syllabus points:

• The requirement of a voluntary act.
• Acts that are considered involuntary (e.g., reflex, convulsion, acts during unconsciousness).
• Liability based on an omission to act.
• The circumstances creating a legal duty to act (e.g., statute, relationship, contract, voluntary assumption of care, creation of risk).
• Possession as a criminal act.

Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, remember to look more closely at that area during your revision.

1. For criminal liability to attach, the defendant generally must have performed:
   1. Any physical act, whether voluntary or involuntary.
   2. A voluntary physical act.
   3. A bad thought coupled with a specific intent.
   4. An omission where a moral duty to act existed.
2. A legal duty to act, the breach of which can result in criminal liability for an omission, can arise from all of the following EXCEPT:
   1. A statute requiring action.
   2. A close familial relationship.
   3. A personal moral code obligating intervention.
   4. A contractual obligation to provide care.
3. A defendant driving a car suffers a sudden, unforeseeable heart attack, loses control of the vehicle, and strikes a pedestrian. Can the defendant be held criminally liable for the pedestrian's injuries based on his driving at that moment?
   1. Yes, because driving is a voluntary act.
   2. Yes, under strict liability principles for operating a vehicle.
   3. No, because the loss of control was due to an involuntary act.
   4. No, unless the defendant had a specific intent to harm the pedestrian.
4. Police find illegal drugs hidden in a backpack on the back seat of a car the defendant is driving. The backpack belongs to a friend, and there is no evidence the defendant knew about the drugs. For a possession offense, the strongest argument for the defendant is that:
   1. Possession requires ownership of the drugs.
   2. There is no voluntary act in driving the car.
   3. There is no awareness of the contraband, so no knowing possession.
   4. Possession crimes always require actual physical holding of the contraband.
5. A parent fails to feed her infant for several days despite having food in the house. The infant dies of malnutrition. Which statement best describes the actus reus issue?
   1. There is no actus reus because the parent did not physically harm the infant.
   2. The omission to feed can be the actus reus because the parent had a legal duty to act.
   3. There is no liability because omissions can never be criminal.
   4. There is no liability unless a statute specifically mentions parents.

Introduction

A fundamental principle of criminal law is that a person cannot be punished for bad thoughts alone. For criminal liability to arise, there must generally be an actus reus—a physical act or, in limited circumstances, an omission to act—committed by the defendant. This act must be voluntary. The actus reus must also concur with the required mental state (mens rea) and, for result crimes, cause the prohibited harm.

Key Term: Actus Reus
The external portion of a crime: a voluntary physical act, a qualifying omission, or possession, together with any required attendant circumstances and results.

Criminal law also rejects liability based on mere status or condition (such as being an alcoholic) and based on thoughts alone. The law focuses on conduct—what the defendant voluntarily does (or in certain settings fails to do).

This article explores:

• The requirement of a voluntary act and what counts as “involuntary.”
• How omissions can sometimes substitute for an act.
• How possession functions as a criminal act.

Throughout, keep two exam questions in mind:

• Is there any voluntary conduct that can serve as the actus reus?
• If the case involves an omission or possession, do the special requirements for those theories of liability appear in the facts?

Voluntary Act Requirement

Criminal liability requires a voluntary act. A voluntary act is a willed bodily movement—the product of the actor’s conscious control.

Key Term: Voluntary Act
A willed bodily movement or muscular contraction, as opposed to a purely reflexive, convulsive, or unconscious movement.

This does not mean the act must be wise, rational, or free from pressure; it need only be under the person’s control in the physical sense. Impulsive acts and acts done under duress are still “voluntary” for actus reus purposes, though duress may be a separate defense.

Two important implications:

• Mere thoughts are never enough; there must be some conduct.
• The law targets acts, not mere status (e.g., being an addict or being homeless).

Key Term: Status Crime
A statute that purports to criminalize a person’s status or condition rather than any conduct. Pure status crimes are unconstitutional; there must be some voluntary act (such as use or possession).

Involuntary Acts

Certain bodily movements are considered involuntary and cannot themselves satisfy the actus reus requirement:

• Reflex or convulsion: Movements produced by an epileptic seizure or sudden spasm.
• Acts during unconsciousness, blackout, or sleep: Movements while genuinely unconscious, sleepwalking, or under certain forms of automatism.
• Movements under hypnosis or resulting from hypnotic suggestion (in many jurisdictions).
• Movements physically forced by another: For example, another person physically pushes the defendant into someone else.

These movements are not the product of the defendant’s will. If the only relevant conduct in the fact pattern is of this type, actus reus is usually missing.

However, on the MBE you must also consider whether an earlier voluntary act can be the basis of liability. Courts often look at the defendant’s conduct over time and may treat the case as involving a voluntary act plus a foreseeable risk.

Prior Fault and “Time-Framing” the Act

Even if the immediate harmful movement is involuntary, the defendant may be liable if:

• The defendant’s earlier voluntary conduct created a known risk of such an episode; and
• The defendant consciously chose to engage in that conduct despite the risk.

Typical examples:

• A driver with a history of uncontrolled seizures drives against medical advice and has a seizure that causes a crash.
• A diabetic who knows that skipping insulin will likely cause loss of consciousness drives anyway and blacks out.

In such cases, the voluntary act is the decision to drive (or to engage in the risky activity) with knowledge of the condition, not the convulsive movement or blackout itself.

Worked Example 1.1

David suffers from severe epilepsy. While having a seizure, his arm flails and strikes Brenda, causing injury. Is David liable for battery based on the act of striking Brenda during the seizure?

Answer:
No. David's arm movement during the seizure was an involuntary convulsive act, not a product of his conscious will. Therefore, the actus reus requirement for battery is not met by the striking itself. Liability could arise only if David had a prior voluntary act that created an unjustified risk (for example, deciding to drive knowing he was likely to seize and endanger others).

Additional Nuances on Voluntariness

A few exam-focused points:

• Intoxication: Movements while voluntarily intoxicated are still voluntary acts. Choosing to drink or use drugs is voluntary, and conduct while intoxicated generally satisfies the actus reus requirement (intoxication may be relevant to mens rea, not to actus reus).
• Coerced acts: If someone threatens the defendant, and the defendant responds by acting (e.g., handing over money at gunpoint), the bodily movement is voluntary. Coercion is analyzed under the defense of duress, not as a failure of actus reus.
• Habitual conduct: Routine acts (like automatically locking a door or steering a car) are treated as voluntary; the fact that the defendant did not think carefully at the moment does not make the act involuntary.

Worked Example 1.2

Sam has a history of sudden, unpredictable heart attacks. His doctor has not warned him against driving and Sam has had no recent symptoms. While driving carefully, Sam experiences a completely unexpected heart attack, loses consciousness, and his car hits a pedestrian.

Answer:
Sam’s loss of control at the moment of the crash is involuntary. There is also no earlier voluntary act that involves prior fault: he had no reason to anticipate an imminent attack or increased risk in driving. On these facts, Sam has not committed a voluntary act that can serve as the actus reus of a crime. On the MBE, this aligns with answer choice (c): no liability because the loss of control was due to an involuntary act.

Thoughts and Status Alone Are Not Enough

The voluntary-act requirement also explains why:

• You cannot be convicted for merely thinking about committing a crime.
• Pure status offenses (e.g., “being a drug addict”) are unconstitutional. The law must focus on conduct like using or possessing drugs, not on the condition of addiction itself.

Omissions as Actus Reus

While criminal law generally punishes affirmative acts, a failure to act—an omission—can sometimes serve as the actus reus of a crime. There is, however, no general duty to rescue or aid a person in danger. A moral obligation alone is not enough.

Key Term: Omission
A failure to act. An omission can constitute the actus reus of a crime only when the law imposes a legal duty to act, the defendant knows the facts giving rise to that duty, and the defendant is physically and reasonably capable of acting.

For liability based on an omission, four elements are usually required:

• A legal duty to act (not merely a moral duty).
• Knowledge of the facts that trigger the duty.
• Actual ability to perform the act (or at least to seek help).
• A failure to act that sufficiently contributes to the prohibited harm.

Legal Duty to Act Required

Key Term: Legal Duty
An obligation imposed by law—not merely by conscience—to take particular action. Legal duties most commonly arise from statutes, contracts, special relationships, voluntary assumption of care, or creating a risk.

A legal duty can arise in several specific circumstances:

• By Statute:
  Some statutes impose an affirmative duty to act, such as:

  • Filing tax returns.
  • Registering as a sex offender.
  • Remaining at the scene of an accident and rendering required assistance.
  • Reporting child abuse or gunshot wounds in certain professions.

  Failure to comply can itself be the actus reus of a standalone offense (e.g., “failure to file”).

• By Contract:
  A contractual obligation to provide care or services can create a duty to act. Examples:

  • A lifeguard employed to safeguard swimmers.
  • A nurse hired to care for a patient.
  • A babysitter or nursing home staff member.

  If the contractual duty is breached and harm results, an omission can satisfy the actus reus element of a relevant crime (e.g., manslaughter).

• By Relationship Status:
  Certain relationships impose a legal duty of care, most notably:

  • Parent–child (especially to minor children).
  • Spouses.
  • Sometimes others in a “custodial” position, like a legal guardian.

• By Voluntary Assumption of Care and Seclusion:
  A person who voluntarily takes charge of a helpless individual and, by doing so, cuts off other potential sources of aid, assumes a duty to act. Liability can arise if that person later abandons the victim or fails to secure help.

• By Creation of Risk:
  If a person, even accidentally, creates a risk of harm to another, they usually have a duty to take reasonable steps to prevent or mitigate that harm. Examples:

  • Knocking someone into a pool.
  • Starting a fire, even negligently.
  • Causing a traffic collision and failing to call for assistance.

In all of these categories, the omission is treated as the actus reus because the law says “you must act” under those circumstances.

Worked Example 1.3

Swimmer is drowning 50 yards from shore. Bystander, an expert swimmer, watches from the beach but does nothing, even though a rescue attempt would pose no risk to Bystander. Swimmer drowns. Can Bystander be held criminally liable for Swimmer's death based on his omission?

Answer:
No. Generally, there is no legal duty to rescue a stranger, even if the rescue could be performed easily and safely. Bystander had no relationship with Swimmer, was not contractually obligated to help, had not assumed care, and did not create the risk. Absent a legal duty, Bystander's failure to act, though morally blameworthy, does not satisfy the actus reus requirement for homicide.

Knowledge and Capability

Even if a legal duty exists, omission liability requires:

• Knowledge of the circumstances:
  The defendant must be aware (or at least practically certain) of the facts that give rise to the duty. If a parent genuinely does not know a child is in danger, the omission to intervene generally cannot be criminal.

• Actual, reasonable ability to act:
  The defendant must be physically and reasonably capable of acting (or at least of seeking help). The law does not require a person to perform the impossible or to take on an unreasonable risk of serious harm to themselves.

Common exam patterns:

• The defendant is physically incapacitated (e.g., unconscious or paralyzed) and cannot rescue; no liability for failing to rescue.
• The defendant can easily call 911 or otherwise obtain help, but does not; that failure can satisfy the duty if a legal duty is present.

Worked Example 1.4

A mother lives alone with her 6‑month‑old baby. She has sufficient food in the house but, out of indifference, intentionally stops feeding the child. The baby dies of malnutrition.

Answer:
The mother has a clear legal duty to care for her minor child arising from their relationship. She knew the child needed food and was physically able to feed the baby. Her omission—failing to feed the baby—can serve as the actus reus of homicide (typically murder or manslaughter, depending on the mental state).

Worked Example 1.5

Driver negligently bumps Cyclist into a shallow creek. Cyclist is conscious but injured and cannot climb out. Driver sees Cyclist in distress but, fearing legal trouble, drives away without calling for help. Cyclist later drowns when the creek unexpectedly floods.

Answer:
Driver’s initial act created the risk to Cyclist. That gives Driver a legal duty to take reasonable steps to protect Cyclist, such as calling emergency services or attempting a safe rescue. Driver’s omission to act after creating the risk can satisfy the actus reus requirement for a homicide offense, assuming causation is proven.

Good Samaritan and “Failure to Rescue” Statutes

Some jurisdictions have enacted “Good Samaritan” or “duty to rescue” statutes that:

• Impose a limited duty on bystanders to render reasonable assistance or call for help in specified emergencies; or
• Protect volunteers from civil liability when they attempt rescue.

For criminal liability based on omission on the MBE:

• Look for explicit statutory language imposing a duty to act; otherwise, assume the traditional common law rule (no general duty to rescue).
• If such a statute is present, breach of that statutory duty can be the actus reus of a specific offense defined by the statute.

Possession as an Act

Possession of contraband (e.g., illegal drugs, unregistered firearms, stolen goods) can itself constitute the necessary criminal act, even though the conduct is largely passive.

Key Term: Possession
Having control over an item. Possession can be actual (physical custody) or constructive (the ability and intent to exercise control over the item even without physical custody).

For possession to be a criminal act, three main components are usually required:

• Control: The defendant must exercise dominion and control over the item.
• Awareness: The defendant is generally required to know that they possess the item (they need not know that the item is illegal, unless the statute so provides).
• Duration: The possession must last long enough that the defendant could have terminated it once aware of the item.

Actual vs Constructive Possession

Key Term: Actual Possession
Direct physical control over an item, such as holding it in one’s hand, having it in a pocket, or carrying it in a bag.

Key Term: Constructive Possession
Control over an item located somewhere the defendant has the power and intent to exercise dominion, such as in a car, house, or storage locker, even when the defendant is not physically touching it.

Examples of constructive possession:

• Drugs hidden in the defendant’s bedroom closet in a house the defendant occupies.
• An illegal firearm locked in a safe to which the defendant has the only key.
• Contraband stored in a locker rented by the defendant, where only the defendant knows the combination.

Joint possession is possible: two or more people can each have constructive possession of the same item if each has the power and intent to control it.

Awareness and Duration

Most possession statutes require that the defendant knowingly possess the prohibited item. Thus:

• A person who genuinely and reasonably believes a package contains lawful goods typically lacks the required awareness of possession of contraband.
• Once the defendant becomes aware of the contraband, the question becomes whether there is time and opportunity to rid themselves of it. If the defendant knowingly retains possession, even briefly, that period ordinarily suffices.

Some statutes impose strict liability and do not require awareness that the item is contraband. Those statutes will say so explicitly; in their absence, assume some level of knowledge is required.

Worked Example 1.6

Police lawfully stop a car driven by Dana. On the back seat is a closed backpack belonging to her passenger. Inside are illegal drugs. There is no evidence that Dana knows about the drugs; she insists she believed the bag contained only school books.

Answer:
On these facts, Dana does not have knowing possession of the drugs. While she has control of the car, there is no evidence she is aware of the contraband in the backpack. Without awareness, there is no knowing possession, so the actus reus (possession) is not established as to her.

Worked Example 1.7

Gary shares an apartment with Roommate. Police search the apartment and find an unregistered handgun hidden under Gary’s bed. Gary admits he knows the gun is there and occasionally uses it for target practice, but claims it “belongs” to Roommate.

Answer:
Gary has actual knowledge of the gun and the ability to exercise control over it; he even uses it. Possession does not require legal ownership, only control. Gary has at least constructive possession and probably actual possession at various times, so the possession element is satisfied.

Summary

Criminal liability generally requires a voluntary physical act (actus reus) by the defendant. Movements resulting solely from reflex, convulsion, unconsciousness, or comparable conditions are considered involuntary and cannot, standing alone, satisfy the actus reus requirement. However, an earlier voluntary act that creates a known risk (such as choosing to drive despite a known seizure disorder) can supply the required conduct.

Liability based on an omission is exceptional. An omission can satisfy the actus reus element only when:

• A specific legal duty to act exists (from statute, contract, relationship, voluntary assumption of care with seclusion, or creation of risk).
• The defendant knows the facts triggering that duty.
• The defendant is physically and reasonably able to act.
• The failure to act sufficiently contributes to the prohibited harm.

There is no general duty to rescue strangers, and moral blameworthiness alone is not enough.

Possession can also constitute the actus reus. To use possession as the required act, the prosecution must typically show:

• Actual or constructive control over the item.
• Awareness of the item’s presence.
• Possession for a sufficient time to allow the defendant to terminate it once aware.

Understanding these doctrines allows you to identify when the actus reus element is present, and just as importantly, when it is missing, which is frequently the deciding issue in MBE criminal law questions.

Key Point Checklist

This article has covered the following key knowledge points:

• Every crime requires an actus reus—a voluntary act, qualifying omission, or possession.
• A voluntary act is a willed bodily movement; reflexes, convulsions, and movements during unconsciousness or true automatism are involuntary.
• Bad thoughts and pure status (such as being an addict) cannot by themselves constitute the actus reus.
• An earlier voluntary act with prior fault (e.g., choosing to drive despite a known medical risk) can sometimes satisfy the act requirement even if the immediate harmful movement is involuntary.
• An omission can constitute the actus reus only when there is a legal duty to act, not merely a moral duty.
• Legal duties arise from statute, contract, special relationships, voluntary assumption of care (with seclusion), or creation of risk.
• Liability for omission requires both knowledge of the circumstances giving rise to the duty and a realistic ability to act or secure help.
• Possession—actual or constructive—can serve as the actus reus, but generally requires control plus awareness and possession for a sufficient duration to terminate it.
• Constructive possession exists when an item is in a place under the defendant’s dominion and control and the defendant intends to exercise control over it, even without physical custody.
• Strict-liability possession statutes are possible but must be clearly indicated; otherwise, assume a knowledge requirement.

Key Terms and Concepts

• Actus Reus
• Voluntary Act
• Status Crime
• Omission
• Legal Duty
• Possession
• Actual Possession
• Constructive Possession