I. CONCURRENCE VERSUS TRACINGIt is hornbook (criminal) law that it is not enough for criminal liability that a defendant did the actus reus of some crime without justification, even if it is also the case that she had a culpable mental state of intention or belief formed in a situation that was not excusing; in addition, the actus reus and the mens rea must in some sense concur. 1 Often courts treat the concurrence of actus reus and mens rea as mere temporal simultaneity, giving rise to the alternative name for the concurrence requirement, namely, the ''simultaneity requirement. '' 2On this view, to be guilty of say first degree murder, defendant must not only do the act of killing and must not only form the intention to kill, but he must have the intent to kill at the same time as he kills. In imposing this concurrence requirement for legal liability, courts assume that the law here is attempting to mirror a basic requirement for moral responsibility for some harm. 31 Notice that the concurrence requirement extends to the defenses as well: if defendant was trying to defend himself (justification) or was under duress (excuse), the threats giving rise to the defenses of selfdefense or to duress must concur with the defendant doing the wrong of killing with the intent to kill. For these defenses to be valid, defendant's intent to kill must be formed while he is under the threat of another, not at some other time or for some other reason. 2 See, e. g. , Elise Sugarman, in her ''Connecting Mens Rea and Actus Reus: Toward a New Theory of Correspondence, '' forthcoming in Journal of Criminal Law and Criminology, 116 (2026), pp. ____, documents how pervasive in Anglo-American criminal law doctrine is the equation of concurrence with temporal simultaneity. 3 As with wrongdoing and culpability themselves, the concurrence requirement is part of the general part of criminal law mirroring that kind of morality commonly known as ''ascriptive'' or ''hypological'' morality. See Moore, ''The Specialness of the General Part of the Criminal Law, '' in Dennis Baker, ed. ,
Michael S. Moore (Thu,) studied this question.