Law Professor Christian Dahlman, of Lund University, Sweden, recently published an article entitled Unacceptable Generalizations in Arguments on Legal Evidence, Argumentation 31, 83-99 (March 2017), which he hopes “will enhance the clarity” of the evidence assessments made by legal decision makers, and “make them more reasoned.” Id. at 86.

The article starts by explaining that every argument regarding legal evidence comes with an inherent generalization, which may or may not be expressly stated.

Some generalizations are so trivial and uncontroversial that judges and jurors do not even think about them as premises in the argument. Other generalizations are problematic, and there are some arguments that trade on generalizations that are unacceptable.


Professor Dahlman spends his time exploring four ways that arguments might trade on unacceptable generalizations.

First, a generalization might be unacceptable simply because it is false. For example, a generalization that at a person with a prior conviction for a similar offense is more likely to be guilty of the charged offense this time around is only a proper generalization if, in fact, accused people with priors are found guilty more often than accused people without. Id. at 90. According to Professor Dahlman, however, that generalization has been shown, empirically, to be untrue. Id. at 90-91; see also Dahlman, C., The felony fallacy, Law, Probability and Risk 14, 229-241 (2015).

Second, a generalization might be improper because it is based on a “non-robust” reference class. This fallacy can be understood by applying “the principle that a generalization should not be accepted if the reference class can be specified in a way that typically changes the probability of [the hypothesis being true.]” Id. at 94. For example, men commit most murders. Therefore, it might be tempting to view the sex of a man accused of murder as being evidence indicating his guilt. But that generalization ought not be accepted because the reference class of “men” is too heterogeneous (“non-robust”). The generalization ought to be rejected if the chance of guilt could be changed by making the reference class more specific (“robust”); say by measuring the accused against reference classes such as: “[other] males with a track record of good behavior,” or “[other] males over 65 with a track record of good behavior.” Id.

Third, a generalization might be unacceptable because it tends to trigger bias among the fact-finder. This basis for rejecting this type of generalization is that “the decision maker[‘s] fears that the generalization, if accepted, will be overestimated and overused.” Id. at 95. As an example, Professor Dalhman points to Federal Rule of Evidence 403 (equivalent to Mil. R. Evid. 403), which permits judges to exclude relevant evidence on the grounds that its probative value is substantially outweighed by the danger of unfair prejudice, confusing the issues, misleading the jury, undue delay, wasting time, or needlessly presenting cumulative evidence.

The last way the article discusses for how an evidentiary generalization might be improper is that it might be morally objectionable “categorically because it discriminates people that belong to a reference class in an unfair way.” Id. at 97. The danger with this sort of fallacy is that if it were permitted to receive judicial sanctioning then, rather than merely impacting a single case, it could put an entire class of people at a systematic disadvantage. Id.

The Supreme Court’s decision in February of this year in Buck v. Davis, 137 S. Ct. 759 (oyez), is illustrative of Professor Dahlman’s last point. In that case, during sentencing, Buck’s own lawyer elicited (perhaps inadvertently) testimony from the defense’s own mitigation expert that Buck’s race (African American) made him a higher risk for violent re-offense. The Supreme Court reversed Buck’s sentence, holding: “it is inappropriate to allow race to be considered as a factor in our criminal justice system” regardless of which party injected race into the trial. 137 S. Ct. at 790. Similarly, in March of this year, in Pena-Rodriguez v. Colorado, 137 S. Ct. 855 (oyez), the Supreme Court held that a criminal defendant has a Constitutional right to pierce the veil of jury deliberations, and to impeach the jury’s verdict, when evidence comes to light suggesting that the jury may have been influenced by racial bias when evaluating the evidence. These are powerful precedents against allowing discriminatory bias to seep into judicial processes, and should serve as a deterrent to improper evidentiary generalizations overall.

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