There are two doctrinal puzzles about accomplice liability in Anglo-American criminal law. One is the puzzle about the mental states required for conviction as an accomplice. Hornbook law has it that accomplice liability is a “specific intent” offense, a requirement that the accomplice have “purpose” and not merely “knowledge,” as those terms are used in the Model Penal Code. Yet, are there two mens rea requirements here, a “primary” mens rea having as its object the aiding of the conduct of another person, and a second requirement having as its object the elements of the underlying crime aided? If so, does the secondary requirement expand or limit the liability otherwise permitted by the primary requirement? What is the relationship between the mens rea required for conviction of guilty principals and the secondary mens rea required for conviction as an accomplice? Does this vary depending on the kind of element (circumstance or result) of the underlying offense involved?

Interesting and important as these mens rea questions are, my focus here is on a second puzzle about accomplice liability, having to do with the actus reus of being an accomplice, not its mens rea. Put generally, what does one have to do in order to be guilty as an accomplice to someone else’s crime? Again, the hornbook law answer (in the ancient language of the common law) is that one must “aid and abet” another’s commission of a crime in order to be guilty as an accomplice to that crime. So one can phrase my inquiry as a question about what it is to aid or abet another to do something.