The crime of conspiracy is traditionally defined as an agreement between two or more persons, entered into for the purpose of committing an unlawful act. At first carefully delimited in scope, conspiracy evolved through a long and tortuous history into a tool employed against dangerous group activity of any sort. The twentieth century in particular has witnessed an expansion of conspiracy law in the face of modern organized crime, complex business arrangements in restraint of trade, and subversive political activity. At the same time, indiscriminate conspiracy prosecutions have sparked great controversy, not only because the vagueness of the concept of agreement and the difficulty in proving it frequently result in convictions with only a tenuous basis for criminal liability, but also because conspiracy law involves a number of extensions of traditional criminal law doctrines. The principal extensions are the following:
- Conspiracy criminalizes an agreement to commit a crime, even though an attempt conviction would not be permitted because of the highly preparatory nature of the act.
- Although conspiracy is now generally limited in most jurisdictions to agreements to commit statutorily defined crimes, traditionally persons agreeing to commit tortious acts, or indeed any acts resulting in "prejudice to the general welfare," could be held liable for conspiracy.
- All conspirators are liable for crimes committed in furtherance of the conspiracy by any member of the group, regardless of whether liability would be established by the law of complicity.
- Contrary to the usual rule that an attempt to commit a crime merges with the completed offense, conspirators may be tried and punished for both the conspiracy and the completed crime.
- Special procedural rules designed to facilitate conspiracy prosecutions can prejudice the rights of defendants. For example, all conspirators may be joined for trial, with resultant danger of confusion of issues and of guilt by association; and rules of evidence are loosened to alleviate the difficulties of proving the existence of a clandestine agreement.
In order better to understand and evaluate these doctrines, it is necessary to examine the elements of the crime of conspiracy. Like most crimes, conspiracy requires an act (actus reus) and an accompanying mental state (mens rea). The agreement constitutes the act, and the intention to achieve the unlawful objective of that agreement constitutes the required mental state.