Ch2. Elements of a Crime — Actus Reus, Mens Rea, and Defenses

Elements Required for Criminal Liability

Three Requirements for Criminal Liability:
① Actus reus: a voluntary act (or qualifying omission)
   that satisfies the conduct element of the offense
② Mens rea: the culpable mental state required by the
   offense definition
③ No valid defense: no justification or excuse that
   negates liability

Liability Framework:
Conduct → Actus reus satisfied? → Mens rea present?
→ No valid defense? → Liability established → Punishment

Attempt:
Defendant took a "substantial step" toward completing
the crime but the offense was not consummated.
Generally punished less severely than the completed crime
(MPC § 5.01; varies by state).

Actus Reus and Mens Rea

Actus Reus:
The physical or external element of the crime.
Must be a voluntary act (MPC § 2.01).
Omissions: liability arises only where there is a legal
duty to act (statute, contract, special relationship,
voluntary undertaking, creation of risk).

MPC Mental States (§ 2.02):
Purpose (intent): conscious object to engage in conduct
  or cause the result
Knowledge: awareness that conduct is of that nature
  or that result is practically certain
Recklessness: conscious disregard of a substantial and
  unjustifiable risk
Negligence: failure to perceive a substantial and
  unjustifiable risk that a reasonable person would
  have perceived

Common Law Equivalents:
Specific intent: intent to achieve a further consequence
General intent: intent (or recklessness) as to the
  act itself
Transferred intent: intent "transfers" when the wrong
  victim is harmed

Justification Defenses

Justifications:
Conduct is not wrongful because it was socially
beneficial or at least permissible under the circumstances.

Self-Defense:
Defendant reasonably believed force was immediately
necessary to protect against unlawful force.
Deadly force: only where defendant reasonably feared
death or serious bodily harm.
Duty to retreat (majority/MPC) vs. Stand Your Ground
(many US states — no duty to retreat if in a place
where lawfully present).

Defense of Others:
Same standard as self-defense; defendant may defend
a third party facing unlawful force.

Necessity (Choice of Evils):
Defendant committed a lesser harm to avoid a greater harm.
MPC § 3.02: harm avoided must clearly outweigh harm
caused; no legislative purpose to exclude the justification.

Consent:
Valid consent by the victim negates some offenses
(e.g., assault, battery in contact sports).
Cannot consent to serious bodily harm in most
jurisdictions.

Law Enforcement / Public Authority:
Officers may use reasonable force in making lawful
arrests; deadly force only where necessary to prevent
escape of a dangerous felon (Tennessee v. Garner, 1985).

Excuse Defenses

Excuses:
Conduct was wrongful but the defendant is not held
responsible because of a personal incapacity.

Infancy:
Children under 7: irrebuttable presumption of incapacity.
Ages 7–14: rebuttable presumption of incapacity.
Ages 14+: treated as adults (varies by state).
Federal juvenile delinquency proceedings available.

Insanity:
M'Naghten test (majority): defendant did not know the
  nature/quality of the act OR did not know it was wrong.
MPC § 4.01: lacks substantial capacity to appreciate
  criminality OR to conform conduct to law.
Guilty but mentally ill (GBMI): conviction with
  treatment mandate.
Insanity defense results in acquittal, not release —
  usually commitment to psychiatric facility.

Diminished Capacity:
Mental disorder negates specific intent (not a full
  defense; reduces degree of offense in some states).

Actio Libera in Causa (Voluntary Intoxication):
Voluntarily induced intoxication generally does NOT
  excuse criminal liability.
May negate specific intent (reduces murder to
  manslaughter in some jurisdictions).

Mistake of Law / Ignorance of Law:
General rule: ignorance of the law is no excuse.
Exception: reasonable reliance on official
  misstatement of law (MPC § 2.04(3)).

Key Concept Cards

Three Elements of Criminal Liability ★★★★★ : Actus reus + mens rea + no valid defense. Missing any one element → no liability. Memory tip: ARM = Actus reus, Rea (mens rea), Missing defense.

Self-Defense Requirements ★★★★★ : Reasonable belief of imminent unlawful force + proportional response. Deadly force only against threat of death/SBH. Memory tip: Reasonable + Imminent + Proportional.

Insanity vs. Infancy ★★★★★ : Insanity = cognitive/volitional incapacity (M’Naghten / MPC). Infancy = age-based incapacity (under 7 irrebuttable; 7–14 rebuttable). Memory tip: Insanity = mental; Infancy = age.

Practice Quiz

Q. What is the difference between purpose and recklessness under the MPC?

Purpose: the defendant’s conscious object is to engage in that conduct or cause that result — the highest culpability level. Recklessness: the defendant is aware of a substantial and unjustifiable risk and consciously disregards it. US criminal law generally requires at least recklessness; strict-liability crimes exist but are disfavored and usually limited to minor regulatory offenses.

Q. How does self-defense differ from the necessity (choice of evils) defense?

Self-defense applies when the threat comes from the unlawful conduct of another person; the defendant may direct force against that person. Necessity applies when the threat comes from circumstances (natural forces, third parties not at fault); the defendant causes harm to an innocent party or property. Necessity requires that the harm avoided clearly outweigh the harm caused, and cannot be used if the legislature has addressed the situation (e.g., a prisoner cannot invoke necessity to escape a burning jail if the legislature has criminalized escape absolutely).