The vast majority of criminal prosecutions in the United States are carried out by the several states in accordance with the laws of the state in question. Historically, the states (with the partial exception of civil-law Louisiana) applied common law rules of "mens rea" similar to those extant in England, but over time American understandings of common law "mens rea" terms diverged from those of English law and from each other. By the late 1950s to early 1960s, the common law of "mens rea" was widely acknowledged to be a slippery, vague, and confused mess. This was one of several factors that led to the development of the Model Penal Code.