State v. Barnum
Supreme Court of Missouri
14 S.W.3d 587 (2000)
Candis West (the victim) snuck out to visit her boyfriend Brandon Srader, but found only his sister, Norma Barnum (defendant), home. A group of teenagers, including Srader's new girlfriend, planned to beat up West; everyone but West and one boy knew the plan. The group drove West to a rural low-water bridge, where three girls beat West severely, breaking her nose and two ribs, then forced her to undress and threw her in the water. Barnum watched, laughed, and encouraged the attackers, at one point shouting for them to "just kill her" and to run her over with the car. The group then left West in the water and drove off. The State of Missouri (plaintiff) charged Barnum with first-degree assault as an accomplice; she was convicted and sentenced to 10 years, then appealed.
Whether accomplice liability may be imposed on a defendant who assists in planning a crime and provides aid or encouragement during its commission.