Conspiracy

Common law conspiracy encompassed agreements to commit an unlawful act. The key word is unlawful: it refers not only to criminal, but also to tortious acts, or even to acts that, in the opinion of a court, result in "prejudice to the general welfare or oppression of an individual of sufficient gravity to be injurious to the public interest" (Commonwealth v. Dyer, 243 Mass. 472, 138 N.E. 206 (1922)). This rule owed its origin to seventeenth-century expansion of the scope of conspiracy, which was stimulated by impatience with the narrow technicalities of medieval law, coupled with a tendency to identify criminality with immorality. It was thought that courts had authority to correct

errors and misdemeanors extra-judicial, tending to the breach of peace, or oppression of the subjects, or to the raising of faction, controversy, debate, or to any manner of misgovernment; so that no wrong or injury, either public or private, can be done, but that it shall be here reformed or punished by due course of law. (Bagg's Case, 77 Eng. Rep. 1271 (K.B. 1616) (Coke))

The doctrine's most significant use in the United States occurred in the early nineteenth century, when many courts sustained criminal-conspiracy prosecutions against unions of workers seeking to organize in order to pressure employers to meet their employment demands by collectively withholding their labor (Wellington, pp. 7–46).

In affording discretion to judges to punish as a crime the group pursuit of any objectives they determined to be against morality and the public interest, the law of conspiracy contravened the classic principle of nulla poena sine lege (no punishment without law). It also went contrary to the principle forbidding ex post facto punishment (criminalizing conduct not previously declared to be criminal). Today, such discretionary criminal liability is vulnerable to constitutional attack as violative of due process of law (Musser v. Utah, 333 U.S. 95 (1948)). Although the common law rule still prevails in some jurisdictions—notably in the federal provision directed against conspiracy to commit "any offense" against or to defraud the United States (18 U.S.C. § 371 (1999))—the modern approach limits the scope of conspiracy to statutorily defined criminal objectives, except where the legislature has identified and prohibited specific kinds of concerted activity.