Criminal law (2022): Scope of criminal liability: Complicity

Complicity is the participation in a completed criminal act of an accomplice, a partner in the crime who aids or encourages (abets) other perpetrators of that crime, and who shares with them an intent to act to complete the crime. A person is an accomplice of another person in the commission of a crime if they purpose the completion of a crime, and toward that end, if that person solicits or encourages the other person, or aids or attempts to aid in planning or committing the crime, or has legal duty to prevent that crime but fails to make an effort to prevent it properly.

Unlike attempt, solicitation, and conspiracy, which are crimes in and of themselves, complicity is not itself a crime but is a way of committing a crime. It also differs from an attempt, solicitation, and conspiracy in that it always depends on that crime having been completed (for example, it is never inchoate). Complicity does not require causation of the crime, merely participating in the commission of the crime. In cases where one is complicit because of a failure to act when one has a duty to act to prevent a crime, complicity differs from omission in that liability for complicity arises from the related to other perpetrators, whereas liability for omission arises from a duty relationship to the victim. 

Common law traditionally distinguished between a "principal" perpetrator who is primarily responsible for a crime and an "accessory" perpetrator who is less responsible. However, modern approaches abandon this distinction, and "a person is legally accountable for the conduct of another when he is an accomplice of the other person in the commission of the crime".

For two persons to be complicit in a crime that does not involve negligence, they must share the same criminal intent; "there must be a community of purpose, partnership in the unlawful undertaking". An accomplice "is a partner in the crime, the chief ingredient of which is always intent". In crimes not involving negligence, there should be evidence that an accomplice had knowledge of the intention of their partner.