Though they have evolved over time, and continue to do so, the elaborate set of procedural and evidentiary rules that specify the types of evidence that are and are not admissible have their origins in English common law (Lempert and Saltzburg, 1982). Restrictions on the use of hearsay, for example, can be traced to common law precedent several hundred years old (Landsman and Rakos, 1991), as can restrictions on the use of character evidence (Wydick, 1987). In the United States, evidentiary rules may also arise from constitutional law. The exclusionary rule ( Weeks v. United States, 1914; Mapp v. Ohio, 1961) restricts the admissibility of evidence gathered in violation of a defendant's rights under the Fourth Amendment. Evidence that violates a defendant's Fifth Amendment privilege against self-incrimination or Sixth Amendment right of confrontation is also excluded.
To further understand the psychology of the evidence rules it is helpful to appreciate some of the purposes judges and rule-makers have had in mind when developing the rules (Lempert, 1977; Lempert and Saltzburg, 1982). One purpose is to screen out evidence that has such limited relevance that it would needlessly prolong the trial
1 In Civil Law systems, including those of most Latin American and continental European nations, there are few exclusionary rules of evidence. Screening of evidence is viewed as unnecessary; the common-sense judgment of fact-finders is viewed as sufficient to afford a reasonable interpretation of the evidence (Damaska, 1997). This confidence in common-sense may rest in part on the use, in most Civil Law systems, of professional or semi-professional fact-finders rather than lay juries. Whether 'trained' fact-finders, such as professional judges, are indeed better than lay jurors at avoiding prejudice and reaching rational conclusions in light of the evidence is an empirical question that has yet to be thoroughly examined.
without contributing to the proper resolution of the matter at hand. Such evidence is ruled inadmissible because it is deemed insufficiently diagnostic to be worth the trouble of hearing.
A second reason for excluding evidence is to screen out material that is prejudicial— that is, evidence that is so likely to bias or mislead the jury that the fact-finding process would be more accurate without it (Kaplan, 1968). One way evidence can be prejudicial is by inflaming the passions of the jury and evoking hostility toward a party without casting much light on the issue at hand. Evidence that a defendant on trial for drunk driving is a pedophile, for example, would likely be judged prejudicial because it is likely to generate more heat than light. Evidence can also be prejudicial if jurors tend to overvalue it, that is, give it more weight than it deserves. Evidence may therefore be excluded because it is thought to be weak or problematic in ways that jurors are unlikely to appreciate. Hearsay evidence, character evidence, and some types of scientific evidence fall into this category.
Finally, some exclusionary rules of evidence are designed to serve specific public policy goals. For example, when a person is sued for negligence after someone has been injured on the defendant's property, evidence that the defendant took subsequent remedial action to prevent future accidents is inadmissible to prove negligence (in the United States, codified in Federal Rule of Evidence 407). This rule is justified in part by 'a fear that if such evidence were admitted, people would be unwilling to take post-accident precautions, to the general detriment of society' (Lempert and Saltzburg, 1977, p. 187). Similarly, evidence that a party offered an out-of-court settlement is inadmissible to prove liability (Federal Rule of Evidence 408) because out-of-court settlements are a favored means of resolving disputes and 'individuals would be reluctant to negotiate compromises if they knew evidence of their offers could be used against them in court' (Lempert and Saltzburg, 1977, p. 191). Rules of privilege also fall into this category. The attorney-client privilege, for example, helps assure that people can communicate freely with their attorneys by preventing disclosure of communications between attorney and client. The marital privilege protects the marital relationship by preventing testimony about communications between spouses. These policy-oriented rules reflect a judgment that accuracy and completeness of fact-finding at trial are less important to society than certain other goals (Lempert and Saltzburg, 1982).
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