The cognitive psychology of mens rea.

I. INTRODUCTION

Actus non facit reum nisi mens sit rea--"the act does not make a person guilty unless the mind is also guilty." (1) Few today would disagree with the maxim; the criminal law has long since rejected the idea that causing harm should be criminal regardless of the defendant's subjective culpability. (2) Still, the maxim begs a critical question: can jurors accurately determine whether the defendant acted with the requisite "guilty mind"? St. Thomas Aquinas was certainly skeptical that such mindreading--as cognitive psychologists call it (3)--is within the ken of mere mortals:

 
      [M]an, the framer of human law, is able to judge only of outward 
   acts; because man seeth those things that appear, according to 1 
   Kings 16.7; but God alone, the framer of the Divine law, is able to 
   judge the inward movement of wills.... (4) 

Given the significant cognitive demands contemporary criminal law imposes on jurors, Aquinas's skepticism seems more prescient than ever. The common law, for example, not only asks jurors to distinguish between seventy-eight different terms for mental states, (5) it fails to define the various mental states consistently--there are seven different definitions of willful alone (6)--and combines them in permutations that defy comprehension. (7) Indeed, the common law is such a mess that no less an authority than Justice Robert Jackson once bemoaned the "variety, disparity, and confusion" of the "requisite but elusive mental element." (8)

The Model Penal Code (MPC), adopted by a majority of states, (9) may actually be worse. To be sure, the MPC takes a far more systematic approach to mens rea, winnowing the common law's seventy-eight mental states to four: purpose, knowledge, recklessness, and negligence. (10) Those mental states are differentiated with such "subtlety and precision," however, that it is an open question whether jurors can accurately distinguish them. (11) Consider, for example, the MPC's three subjective mental states: a person acts "purposely" if his "conscious object" is to bring about a particular result; (12) acts "knowingly" if he is "aware that it is practically certain" that his conduct will lead to the result; (13) and acts "recklessly" if he "consciously disregards a substantial and unjustifiable risk" that his conduct will cause the result. (14) Those are fine distinctions, to say the least. Little wonder, then, that scholars have described the MPC as an "elaborate set of precise rules whose operability depends on the jury's willingness"--to say nothing of their ability--"to make artificial characterizations." (15)

The MPC's idiosyncratic definition of negligence only further complicates mindreading. Negligence is defined as the defendant acting "when he should be aware of a substantial and unjustifiable risk" that his conduct will lead to a particular result. (16) Common law negligence is not actually a mental state; the reasonable-person standard "is determined and applied without reference to what the actor was thinking at the moment." (17) The MPC, by contrast, subjectivizes negligence, once again requiring jurors to read the defendant's mind: whether the defendant's conduct was unreasonable must be determined "considering the nature and purpose of his conduct and the circumstances known to him" at the time of the crime. (18)

Nor is that all. Although the common law at least limited each offense to a single mental state, (19) the MPC permits different mental states to apply to different material elements of an offense--what is known as "element analysis." (20) Misdemeanor indecent exposure is an example: "A person commits a misdemeanor if, for the purpose of arousing or gratifying sexual desire ... he exposes his genitals under circumstances in which he knows his conduct is likely to cause affront or alarm." (21) Thus defined, a jury must determine both whether the defendant acted knowingly (with regard to his conduct's potential to cause affront or alarm) and whether he acted purposely (with regard to arousing or gratifying sexual desire)--a far more complicated mindreading task than determining a single mental state.