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Because the essence of conspiracy is criminal agreement, many definitions of the crime only require an agreement to commit a crime. Critics maintain that contemporary conspiracy definitions set the threshold of crime too early, essentially punishing thoughts rather than conduct. Supporters retort that the early threshold of criminality set by conspiracy is necessary to meet the special dangers posed by collective criminal action.


At common law and until recently in many states, conspiracy was defined as an agreement of two or more individuals to commit a criminal or unlawful act or a lawful act by unlawful means.[3] No conduct other than the agreement itself was required. (Remember that words alone are a type of conduct that can satisfy the actus reus requirement for a crime. See Chapter 3.) Today many (but not all) statutory definitions of conspiracy do require that at least one conspirator take an overt act in furtherance of the conspiracy before the crime is committed.[4] Some states require that one conspirator must take a “substantial act” in furtherance of the conspiracy, pushing the threshold of criminality much closer to the target offense. The Model Penal Code requires an overt act unless the object crimes are serious felonies.[5]

The Common Law

The common law and early statutory definitions of conspiracy did not limit the object of the agreement to crimes. Rather, they included any act that was unlawful or against public policy or even lawful acts committed by unlawful means. These open-ended definitions created uncertainty in the criminal law. Criminal responsibility could attach for agreeing to do something with another (such as charging usurious rates of interest[6] or agreeing to bargain for wages as a group[7]) that, if done alone, would not be a crime. In short, common law conspiracy permits conviction for acts that are not expressly made criminal, creating serious risk of ex post facto punishment.

Thus, in the English case Shaw v. Director of Public Prosecutions,[8] the defendant’s conviction for “conspiracy to corrupt public morals” for agreeing with others to publish a directory for prostitutes was upheld by the House of Lords even though prostitution was not a crime. A statute containing such a broad definitional term would probably be found unconstitutional in the United States as void for vagueness. (See Chapter 1.)

[3] A husband and wife could not commit a conspiracy at common law because a married couple was considered to be one person under the law. This is no longer the law in most jurisdictions.
[4] The general federal conspiracy statute, 18 U.S.C. 371, expressly requires proof of an overt act. But the Supreme Court held in United States v. Shabani, 513 U.S. 10 (1994), that 21 U.S.C. 846, the federal drug conspiracy statute, which is silent about an overt act, does not require one. The Court concluded that congressional silence concerning an overt act indicates that Congress intended to adopt the common law definition of conspiracy for drug conspiracies.
[5] MPC §5.03(5).
[6] Commonwealth v. Donoghue, 250 Ky. 343, 63 S.W.2d 3, 89 A.L.R. 819 (1933).
[7] People v. Fisher, 14 Wend. 2 (N.Y. 1835) (union members who organized to raise wages and refused to work until an employee working below union wages was discharged were found guilty of conspiracy against trade and commerce).
[8] [1962] A.C. 220 (Eng.).

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