Under common law, conspiracy occurs when two or more people agree to do something illegal. The government may charge conspiracy to commit either felonies or misdemeanors. Most states allow an individual to be convicted of both the underlying crime, and a separate charge of conspiracy to commit that crime, with a separate penalty. However, states differ in their treatment of conspiracy; those charged with conspiracy should seek legal counsel.
Elements of Conspiracy
Conspiracy is an inchoate crime, meaning that it's a complete crime in itself, even if the underlying crime was not completed. Common law conspiracy requires at least two elements. First, there must be an agreement between two persons to accomplish either an illegal purpose, or a legal purpose by illegal means. (Note that modern courts generally require an agreement to commit a crime). Second, both parties must have actually intended to enter into this agreement. The majority of jurisdictions have also established a third requirement: that at least one conspirator performs an act in furtherance of the conspiracy (acts that prepare for performance of the conspiracy are generally sufficient).
The agreement between two or more people to conspire does not need to be expressed; it may be implicit or shown by action. The common law standard is "agreement between two or more guilty minds;" this standard allows corporations to be conspirators with individuals or other corporations. Most courts also embrace some form of the Wharton rule, which holds that where an underlying crime requires two or more people to complete, parties additional to those two people must agree for a conspiracy to exist.
Courts require two different forms of intent to prove conspiracy. Each individual defendant must intend to agree, and each must intend to achieve the objective of the conspiracy. Courts will not find intent to conspire merely because a party had knowledge of the crime to be committed . The majority of states allow a conspiracy charge even if a defendant didn't know the plan was illegal; the minority do require knowledge of criminal end (this requirement is known as the corrupt motive doctrine).
If all other parties to an alleged conspiracy are acquitted, the final defendant will usually be able to use that fact as a defense against his own conspiracy charge. Withdrawal from the conspiracy before completion will generally only be a valid defense if the defendant helped to keep the conspiracy from succeeding. However, such withdrawal will insulate the defendant from liability for the other conspirators' acts, including the underlying crime, but he must give other conspirators notice in time for them to decide not to go through with it.
Conspiracy should be distinguished from attempt, another inchoate crime. Attempt is an act committed with criminal intent that doesn't complete a crime. Unlike conspiracy, attempt requires only a single perpetrator, and generally requires a "substantial step" toward completion of the crime. Conspiracy, in many jurisdictions, does not require such a substantial step, only the agreement and intent to agree.