Common law
The common lawso named because it was "common" to all the king's courts across Englandoriginated in the practices of the courts of the English kings in the centuries following the Norman Conquest in 1066. The British Empire spread the English legal system to its historical colonies, many of which retain the common law system today. These "common law systems" are legal systems that give great precedential weight to common law, and to the style of reasoning inherited from the English legal system.
Mens rea
The levels of "mens rea" and the distinction between them vary between jurisdictions. Although common law originated from England, the common law of each jurisdiction with regard to culpability varies as precedents and statutes vary.
The vast majority of criminal prosecutions in the United States are carried out by the component 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.