Christian Dahlman

Arguments on legal evidence rely on generalizations, that link a certain circumstance to a certain hypothesis and warrants the claim that the circumstance makes the hypothesis more probable. Some generalizations are acceptable and others are unacceptable. A generalization can be unacceptable on at least four different grounds. A false generalization is unacceptable because membership in the reference class does not increase the probability of the hypothesis. A non-robust generalization is unacceptable because it uses a reference class that is too heterogeneous. A bias-triggering generalization is unacceptable because decision makers are inclined to overestimate the evidentiary value of membership in the reference class. A discriminating generalization is unacceptable because it puts members in the reference class in an unfair disadvantage. Research funded by the Swedish Research Council (Vetenskapsrådet).

This article is concerned with criminal trials where the defendant is a convicted felon with a prior conviction for the same offence. The article investigates a probabilistic argument that I will call the felony argument. According to the felony argument, the fact that the defendant has been previously convicted for the same offence has probative value in the present case, and increases the probability that the defendant is guilty. The article analyses the felony argument in Bayesian terms to pinpoint under what circumstances it is correct, and under what circumstances it comes to the wrong conclusion. This analysis shows that the felony argument is correct if the probability that a guilty defendant has been previously convicted for the same offence is higher than the probability that an innocent defendant has been previously convicted for the same offence. The problem with the felony argument is that the latter probability is not as low as one might think. It is not unlikely that an innocent defendant has been previously convicted for the same offence. In cases where the defendant is innocent, the cause behind the wrongful prosecution is often related to prior conviction, e.g. when photos of people with a prior conviction for the same offence are presented to an eyewitness and the witness mistakenly identifies a person who has nothing to do with the present case. Prior conviction for the same offence is used by the police as a criterion for selecting suspects, and this selection bias increases the probability that a defendant who is innocent has a prior conviction for the same offence. The defendant in a criminal trial is a person who belongs to a very special sample, the sample of people selected by the police as suspects and then selected for prosecution. An assessment of the probability that he/she is guilty, given a prior conviction for the same offence that does not take into account that he/she belongs to this sample, commits an error, that I call the felony fallacy.

The standard of proof in criminal trials should require that the evidence presented by the prosecution is robust. This requirement of robustness says that it must be unlikely that additional information would change the probability that the defendant is guilty. Robustness is difficult for a judge to estimate, as it requires the judge to assess the possible effect of information that the he or she does not have. This article is concerned with expert witnesses and proposes a method for reviewing the robustness of expert testimony. According to the proposed method, the robustness of expert testimony is estimated with regard to competence, motivation, external strength, internal strength and relevance. The danger of trusting non-robust expert testimony is illustrated with an analysis of the Thomas Quick Case, a Swedish legal scandal where a patient at a mental institution was wrongfully convicted for eight murders.

Arguments on legal evidence rely on generalizations, that link a certain circumstance to a certain hypothesis and warrants the claim that the circumstance makes the hypothesis more probable. Some generalizations are acceptable and others are unacceptable. A generalization can be unacceptable on at least four different grounds. A false generalization is unacceptable because membership in the reference class does not increase the probability of the hypothesis. A non-robust generalization is unacceptable because it uses a reference class that is too heterogeneous. A bias-triggering generalization is unacceptable because decision makers are inclined to overestimate the evidentiary value of membership in the reference class. A discriminating generalization is unacceptable because it puts members in the reference class in an unfair disadvantage. Research funded by the Swedish Research Council (Vetenskapsrådet).

Abstract.  The analysis of legal statements that are made from an “internal point of view” must distinguish statements where legal obedience is accepted from statements where legal obedience is only assumed. Statements that are based on accepted obedience supply reasons for action, but statements where obedience is merely assumed can never provide reasons for action. It is argued in this paper that John Searle neglects this distinction. Searle claims that a statement from the internal point of view provides the speaker with reasons for actions that are “self-sufficient” in the sense that they are independent of the speaker's beliefs and desires. This claim is mistaken. A statement that is based on assumed obedience is self-sufficient, but does not give reasons for action. A statement that is based on accepted obedience gives reasons for action, but these reasons are not self-sufficient.

Abstract. According to Svein Eng there are propositions concerning the law which are descriptive as well as normative, but cannot be separated into one descriptive and one normative proposition. Eng calls these propositions “fused” (“sammensmeltede”). In Eng's theory a proposition with “fused modality” is partly descriptive and partly normative, but cannot be classified as a separable combination of a claim about what the law “is” and a claim about what the law “ought to be.” In a “fused” proposition modality is a question of “degree.” The purpose of this article is to show why Eng's theory should be rejected. The introduction of “fused modality” adds nothing of value to legal theory. Eng claims to have discovered a class of propositions not previously accounted for, but this is not the case. The lawyer Eng talks about as making a “fused” proposition is simply a lawyer logically confused.