Christian Dahlman

The function of Opinio Juris in customary international law

Th is article claims that the requirement of opinio juris in the formation of customary international law means that a general practice must be generally accepted among states to become customary law. Th e article argues that opinio juris serves an important function. It prevents generally unwanted general practice from becoming customary law.

Fallacies in ad hominem arguments

Arguments ad hominem are common in political debates, legal argumentation and everyday conversations. In this article, we propose a general definition of ad hominem arguments. An argument ad hominem is an argument that makes a claim about the reliability of a person in the performance of a certain function, based on some attribute relating to the person in question. On the basis of this definition, we examine the different ways that ad hominem arguments can go wrong, and classify them as seven different ad hominem fallacies: false attribution, irrelevant attribute, overrated effect, reliability irrelevance, irrelevant person, insufficient degree and irrelevant function. The various fallacies are illustrated with examples from politics, law and everyday life.

Reasons Pro et Contra as a Debiasing Technique in Legal Contexts

Although legal contexts are subject to biased reasoning and decision making, to identify and test debiasing techniques has largely remained an open task. We report on experimentally deploying the technique “giving reasons pro et contra” with professional (N = 239) and lay judges (N = 372) at Swedish municipal courts. Using a mock legal scenario, participants assessed the relevance of an eyewitness’s previous conviction for his credibility. On average, both groups displayed low degrees of bias. We observed a small positive debiasing effect only for professional judges. Strong evidence was obtained for a relation between profession and relevance-assessment: Lay judges seemed to assign a greater importance to the prior conviction than professional judges did. We discuss challenges for future research, calling other research groups to contribute additional samples.

Prototype Effect and the Persuasiveness of Generalizations.

An argument that makes use of a generalization activates the prototype for the category used in the generalization. We conducted two experiments that investigated how the activation of the prototype affects the persuasiveness of the argument. The results of the experiments suggest that the features of the prototype overshadow and partly overwrite the actual facts of the case. The case is, to some extent, judged as if it had the features of the prototype instead of the features it actually has. This prototype effect increases the persuasiveness of the argument in situations where the audience finds the judgment more warranted for the prototype than for the actual case (positive prototype effect), but decreases persuasiveness in situations where the audience finds the judgment less warranted for the prototype than for the actual case (negative prototype effect).

Appeal to expert testimony - A Bayesian approach

In this chapter, we offer a Bayesian model for evaluating expert testimony in the court room. Statements from a putative expert are difficult for a legal decision maker to assess, as the legal decision maker – who lacks expert knowledge on the subject issue – must distinguish between experts that are highly reliable and experts that are less reliable. A methodology for the assessment of the expert testimony has been suggested previously, in the works of Walton and Goldman, and we develop this methodology further, using a Bayesian approach to reliability assessment. The reliability of an expert can be questioned on different grounds (lack of competence, bias and lack of motivation), and we clarify different effects that these grounds can have on the expert’s reliability.