An individual may be culpable for the crime of another if he intentionally abbets, encourages, solicits, advises or procures the commission of the crime by physical conduct or psychological influence. Generally, a failure to prevent another from committing a crime will not trigger accomplice liability. However, that ommission could be a factor in determining whether psychological influence existed.
Under the Pinkerton Doctrine  an accomplice can be liable for the criminal acts of another as long as the criminal act falls within the scope of the conspiracy or is a foreseeable consequence of a conspiracy.
In the United States, the mens rea requirement for accomplice liability is normally purpose, though some courts state that knowledge will suffice. The Model Penal Code states that accomplice liability will exist if a defendant assists another "with the purpose of promoting or facilitating the commission fo the offence . If the primary actor can be convicted because the mens rea requirement is only recklessness or negligence, then the same level may apply to accomplice liability.
There are two primary defenses to accomplice liability:
- Protected Class Exemption - First, if the accomplice is a member of the class the crime is intended to protect, accomplice liability does not exist. For instance, a 13 year old girl will not be culpable for statutory rape under an accomplice liability theory.
- Abandonment - An accomplice may abandon a conspiracy and thereby eliminate accomplice liability. In order to do so, the accomplice must generally 1) notify the principal that he or she is withdrawing from the conspiracy and 2) take some action to neutralize whatever steps he took to assist in the crime.
- Pinkerton v. United States, 328 U.S. 640 (1946)
- MPC Section 2.06(3)(a)