[A] Common law – One is an accomplice in the commission of an offense if he intentionally assists another to engage in the conduct that constitutes the crime. Accomplice activity may include aiding, abetting, encouraging, soliciting, advising, and procuring the commission of the offense.
Accomplice liability is derivative in nature. In general, the accomplice may be convicted of any offense committed by the primary party with the accomplice’s intentional assistance. Most jurisdictions extend liability to any other offense that was a natural and probable consequence of the crime solicited, aided or abetted.
[B] Model Penal Code – The Code rejects the common law natural-and-probable-consequences rule. Thus, an accomplice may only be held liable under the Code for acts that he purposefully commits.
[A] Principal in the First Degree – A "principal in the first degree" is one who, with the mens rea required for the commission of the offense: (1) physically commits the acts that constitute the offense; or (2) commits the offense by use of an "innocent instrumentality" or "innocent human agent." The innocent-instrumentality rule provides that a person is the principal in the first degree if, with the mens rea required for the commission of the offense, he uses a non-human agent (e.g., a trained dog) or a non-culpable human agent to commit the crime.
[B] Principal in the Second Degree – A "principal in the second degree" is one who intentionally assisted in the commission of a crime in the presence, either actual or constructive, of the principal in the first degree. A person is "constructively" present if he is situated in a position to assist the principal in the first degree during the commission of the crime, e.g., serving as a "lookout" or "getaway" driver outside a bank that the principal in the first degree robs.
[C] Accessory Before the Fact – An "accessory before the fact" is one who is not actually or constructively present when the crime is committed; often such person solicits, counsels, or commands (short of coercing) the principal in the first degree to commit the offense.
[D] Accessory After the Fact – An "accessory after the fact" is one who, with knowledge of another’s guilt, intentionally assists the him to avoid arrest, trial, or conviction. The conduct of the accessory after the fact occurs after the completion of the crime. If an accomplice is involved prior to the completion, i.e., up to the point when the principal in the first degree has reached a place of temporary safety, the accomplice is in fact a principal in the second degree. Today, nearly all jurisdictions treat the offense of accessory after the fact as separate from, and often less serious than, the felony committed by the principal in the first degree.
[A] Common Law
 Types of Assistance – An accomplice is a person who, with the requisite mens rea, assists the primary party in committing an offense. Generally speaking, there are three basic types of assistance:
|1.)||assistance by physical conduct (e.g., furnishing an instrumentality to commit an offense, "casing" the scene in advance, locking the door to keep an assault victim from escaping, or driving a "getaway" car from the scene of the crime);|
|2.)||assistance by psychological influence (e.g., incitement, solicitation, or encouragement); and|
|3.)||assistance by omission (if there exists a duty to act). A person is not an accomplice simply because he knowingly fails to prevent the commission of an offense, but such failure to act may serve as a critical factor in determining that he assisted by psychological influence.|
 Amount of Assistance Required – A person is not an accomplice unless his conduct (or omission) in fact assists in the commission of the offense. However, the degree of aid or influence provided is immaterial; even trivial assistance suffices. Furthermore, a secondary party is accountable for the conduct of the primary party even if his assistance was causally unnecessary to the commission of the offense.
 The Pinkerton Doctrine – In Pinkerton v. United States, 328 U.S. 640 (1946), two parties conspired to violate certain provisions of the Internal Revenue Code and thereafter, while one co-conspirator was in prison for unrelated reasons, the other carried out the plan. Emerging from this case is the "Pinkerton doctrine" under which a party to a conspiracy is responsible for any criminal act committed by an associate if it:
|1.)||falls within the scope of the conspiracy; or|
|2.)||is a foreseeable consequence of the unlawful agreement.|
[B] Model Penal Code – A person is guilty of an offense if he commits it "by his own conduct or by the conduct of another person for which he is legally accountable, or both." Accomplice liability is founded on:
(1) Accountability through an innocent instrumentality – The Code explicitly provides that the innocent-instrumentality doctrine applies only if one causes another to engage in the conduct in question.
(2) Accomplice accountability – One is an accomplice if, with the requisite mens rea, he solicits, aids, agrees to aid, or attempts to aid in the planning or commission of the offense, or has a legal duty to prevent the commission of the offense, but makes no effort to do so. [MPC § 2.06(2), (3)(a)]
(3) Miscellaneous accountability – Legislatures may enact special laws of accomplice liability, e.g., prohibiting the aiding and abetting of a suicide attempt, [MPC § 210.5(2)] or criminalizing the knowing facilitation of a prison escape. [MPC § 242.6]
The Model Code rejects the Pinkerton doctrine of conspiratorial liability. Thus, a person is not accountable for the conduct of another solely because he conspired with that person to commit an offense. The liability of one who does not personally commit an offense must be based on accountability through an innocent instrumentality, accomplice accountability, or miscellaneous accountability.
[A] Common and Statutory Law
 "Intent" – The mens rea of accomplice liability is usually described in terms of "intention." As with the crime of conspiracy, however, there is considerable debate regarding whether a person may properly be characterized as an accomplice if he knows that his assistance will aid in a crime, but he lacks the purpose that the crime be committed. Most courts, however, hold that a person is not an accomplice in the commission of an offense unless he shares the criminal intent with the principal.
 Recklessness and Negligence – Although courts and statutes frequently express the culpability requirement for accomplice liability in terms of "intent," the majority rule is that accomplice liability may nevertheless attach in cases of crimes involving recklessness or negligence.
[B] Model Penal Code – The Code person resolves the common law ambiguity as to whether complicity requires purpose or mere knowledge of the consequences of their conduct. Under the Code, accomplice liability exists only if one assists "with the purpose of promoting or facilitating the commission of the offense." [MPC § 2.06(3)(a)]
Accomplice liability may also be found in cases involving recklessness or negligence when causing a particular result is an element of a crime:
|1.)||he was an accomplice in the conduct that caused the result; and|
|2.)||he acted with the culpability, if any, regarding the result that is sufficient for commission of the offense. [MPC § 2.06(4)]|
[A] Common Law – At common law, an accessory could not be convicted of the crime in which he assisted until the principal was convicted and, with the limited exception of criminal homicide, could not be convicted of a more serious offense or degree of offense than that of which the principal was convicted.
[B] Modern Rule – Today, the majority rule is that a conviction (or even a prosecution) of the principal in the first degree is not a prerequisite to the conviction of a secondary party. Non-prosecution of the principal might result from any one of numerous factors extraneous to his guilt (e.g., death, flight from the jurisdiction, or immunity from prosecution), and thus, does not in itself prove that a crime did not occur.
Furthermore, even if the principal is prosecuted but acquitted on the basis of an excuse defense, his acquittal should not bar a prosecution and conviction of a secondary party to whom the excuse does not extend. An acquittal on the ground of an excuse means that the actions of the primary party were wrongful, but that he was not responsible for them because of the excusing condition. However, since accomplice liability is derivative, there must be proof at the accomplice’s trial of the principal’s guilt.
An accomplice or accessory may be convicted of a more serious offense than is proved against the primary party.
[C] Model Penal Code – An accomplice in the commission of an offense may be convicted of a crime, upon proof of its commission by another person, regardless of whether the other person is convicted, acquitted, or prosecuted. Furthermore, an accomplice may be convicted of a different offense or different degree of offense than is the primary party. [MPC § 2.06(7)] The Code also expressly provides that a person who is legally incapable of committing an offense personally may be held accountable for the crime if it is committed by another person for whom he is legally accountable. [MPC § 2.06(5)]
[A] Common Law
 Legislative-Exemption Rule – A person may not be prosecuted as an accomplice in the commission of a crime if he is a member of the class of persons for whom the statute prohibiting the conduct was enacted to protect. For example, in a case of statutory rape, an underage female who engages in sexual intercourse cannot be prosecuted as a secondary party to her own statutory rape since the law was enacted to protect young females from immature decisions regarding sex.
 Abandonment – As with the law of conspiracy, many courts hold that a person who provides assistance to another for the purpose of promoting or facilitating the offense, but who subsequently abandons the criminal endeavor, can avoid accountability for the subsequent criminal acts of the primary party. The accomplice must do more than spontaneously and silently withdraw from the criminal activity. He must communicate his withdrawal to the principal and attempt to neutralize the effect of his prior assistance.
[B] Model Penal Code – A person is not an accomplice in the commission of an offense if:
(1) he is the victim of the offense;
(2) his conduct is "inevitably incident" to the commission of the offense;
(3) he terminates his participation before the crime is committed, and:
a) neutralizes his assistance;
b) gives timely warning to the police of the impending offense; or
c) attempts to prevent the commission of the crime.