Mens Rea by the Numbers by Gideon Yaffe

Gabrielle Johansson
3 min readOct 7, 2020


Type 1

In the article entitled “Mens Rea by the Numbers” Yaffe discusses how the implementation of mens rea provisions — which would consider mental states such as intent, neglect, or recklessness — into laws fares in practical ways. numerical argument for consideration and the addition of mens rea in the process of conviction and sentencing fare in practical ways. The concept at the center of this discussion is the numerical argument.

The numerical argument questions whether making the defendant’s mens rea, or lack thereof, a key feature of in the defendant’s case would increase or decrease the amount of guilty verdict’s acquittals. In other words, at the center of this article is a question of whether it would be more just to continue the legal system mostly in the absence of consideration of mens rea or more just to pass a bill which reforms the criminal justice system to include mens rea.

Advocates of the mens rea provision say that it would decrease the amount of “unjust verdicts in corporate cases” — what Yaffe calls false positives: defendants who are convicted without mens rea. Meanwhile, opponents say that the mens reaa provision would reduce the amount of true positive cases and false acquittals because proving mens rea is not easy and sometimes impossible to prove (Yaffe 393).

Yaffe starts out with a relatively uncontroversial claim: “we should not punish without mens rea” (395). In introducing the many complications of the mens rea provision that could unfold in all likelihood, Yaffe intends to illustrate how his relatively uncontroversial claim becomes a matter of controversy and disagreement.

At the root of the issue of the mens rea provision and the numerical argument is the assertion that mens rea must be present to justify criminalization and the assumption that if mens rea must be proved in order to criminalize conviction rates and acquittal rates would flip flop. The controversy surrounds whether it should be necessary to prove the presence of mens rea which would likely make “conviction rates go down, that is, if the prosecution must prove mens rea, and acquittal rates go up” (Yaffe 398). There are clear disadvantages to this, most obviously in cases of sexual crimes against minors: how exactly do you prove mens rea? Knowledge of their age? Intention? Recklessness? At that point, the major disadvantage of mens rea provision is the fact that the question of whether someone will be sentenced boils down to, not what they did, but their word against the courts/juries. Yaffe asks precisely this: why should knowledge (one state of the mens rea) be “such an enormous factor as to make the mismatch between acquittal and culpability to egregious?” (405).

Yaffe continues: “Adopting a regime [the bill] in which some with mens rea are acquitted of sex crimes against children, when an alternative regime that would have convicted them was available, is the performance of an act that makes those crimes worse by making them more wrongful” (406). Though mens rea is important as a feature of criminalization it obviously might make things worse if it were necessary to prove mens rea and risk false acquittals. There is thus room for a mens rea provision on more of a case by case basis because it would probably be wrong for the reasons described above to pass a law which radically reforms the criminal justice system to introduce mens rea provision across the board.

Yaffe, G. Mens Rea by the Numbers. Criminal Law, Philosophy 12, 393–409 (2018).