(c) Before appointing an expert, the prosecutor should consider the expert`s references, relevant professional experience and reputation in the field. The prosecutor should also investigate the background and credentials of an expert testifying on possible impeachment issues. Before proposing an expert as a witness, the prosecutor should consider the scientific acceptance of the theory, method or particular conclusions about which the expert would testify. (a) The prosecution should be aware of the legal standards applicable to jury selection and train prosecutors to comply with them. The prosecutor should prepare himself to effectively exercise the prosecutorial function in jury selection, including the exercise of just cause and compelling challenge. The prosecution should also be informed of the selection and convening procedure for the jury panel and alert the court to legal loopholes. The question of whether or not mens rea should be necessary to give rise to criminal liability is rarely considered sufficient. The widespread belief that we should not tolerate thought crimes leads most authors to argue that there should be an actus reus element for every crime. Paradigmatically, this element is fulfilled only if (D) acts in a way that causes an outcome, such as death or property damage or fear of violence. Of course, this paradigm allows for a number of exceptions. In addition to immature crimes such as attempts or conspiracies, most criminal justice systems provide liability for certain omissions.
Imagine (D) seeing (V) drown in a shallow pond and decides to do nothing. There is no previous connection between (D) and (V). If the pond is in London, (D) does not commit a crime. Move the drama to Paris and we ourselves have a crime. As this example shows, academics and legal systems still disagree on the positive obligations that criminal law should impose (Alexander, 2002; Ashworth, 2015). (d) The obligation to investigate and disclose such information shall remain in force throughout the prosecution. There is a deeper problem with the probabilistic conception of the standard of proof. There does not appear to be a satisfactory interpretation of likelihood that corresponds to the forensic context. The only plausible candidate is the subjective sense of probability, according to which probability is interpreted as the strength of faith. The evidence is sufficient to meet the legal standard of proof for a contested question of fact – for example, it is sufficient to justify the positive finding of fact that the accused killed the victim – if the investigator, after considering the evidence, is sufficiently satisfied that the accused killed the victim.
A guide to dealing with proofs and forming beliefs can be found in a mathematical theorem known as the Bayesian theorem. This is the method by which an ideal rational investigator would revise or update their beliefs in light of new evidence. [19] Returning to our previous hypothetical scenario, suppose the investigator initially believes that the probability that the accused is guilty is 1:1 (“previous chances”) or, in other words, that there is a probability of 0.5. The investigator then receives evidence that Group A blood was found at the scene and that the defendant has Type A blood. Fifty percent of the population has this blood type. In the Bayesian approach, the posterior odds are calculated by multiplying the previous odds (1:1) by the probability ratio (which, as we saw in section 2.1.2 above, is 2:1). The investigator`s belief in the likelihood of guilt should now be revised to 2:1; the probability of guilt is now increased to 0.67 (Lempert, 1977). Bentham was a fierce critic of exclusion rules. He was very much in favour of “freedom of evidence”, understood as free access to information and the absence of formal rules restricting such access (Twining 2006:232, n 65).
The direct object of judicial proceedings is the “legality of the decision”, i.e. the correct application of substantive law to truthful findings of fact. The exclusion of relevant evidence – evidence that can shed light on the truth – is detrimental to this objective. Therefore, no relevant evidence should be excluded; the only exceptions he would allow are if the evidence is superfluous or would result in enormous delays, costs, or hassle (Bentham 1827: Book IX; Bentham 1825: Book VII; Twining 1985: chap. 2). Bentham`s argument has been challenged on several fronts. He is said to have overestimated the search for truth, underestimated procedural fairness and rights, and placed too much trust in public servants by underestimating the risk of abuse when given unregulated discretion (Twining 1985:70-71). Even if the theory is correct, it does not necessarily follow that the exclusion rules should be abolished once the jury system is abolished. Judges may be just as sensitive to the same cognitive and other impairments as the jury, and there may be an additional risk that judges will overestimate their own cognitive and intellectual abilities in their professional field. Therefore, the constraints of legal rules remain necessary (Schauer 2006: 185-193). But the effectiveness of these rules in a non-jury system is questionable. The procedural reality is that judges must be exposed to evidence in order to decide their admissibility.
Given that a judge cannot reasonably be expected to erase the evidence from his mind once he has decided to exclude it, it seems unnecessary to exclude the evidence; We might as well let the evidence in and allow the judge to give the evidence the probative value it deserves (Mnookin 2006; Damaška, 2006; cf. Ho 2008: 44-46). Wigmore takes a different position. He counters to Thayer that relevance is a legal concept. Its claim is twofold. The first is that the legal relevance of the evidence requires “a degree of probative value generally higher than that which would be required in ordinary reasoning”: a second criticism of the theory of relative plausibility is that, despite the alleged use of the “conclusion for the best explanation” argument, judgment is not controlled by the best explanation. For example, even if the prosecution`s hypothesis is better than the defence`s hypothesis, neither can be very good. In these circumstances, the court must reject the prosecutor`s hypothesis, even if it is the best alternative (Laudan 2007). One proposed mitigation of this criticism is to require a number of epistemic efforts that the trier of fact must make (for example, by being sufficiently diligent and thorough) to construct the set of hypotheses from which the best one should be chosen (Amaya 2009: 155).