the Nature of Evidence
Evidence is any statement or material object from which reasonable conclusions can be drawn. It is a broad category embracing anything perceptible to the five senses including documents, exhibits, facts agreed to by both sides, and the testimony of witnesses. Evidence in a criminal trial concerns the intent, motive, means, and opportunity to commit a crime.
In general, evidence is divided into two categories： circumstantial and physical. Circumstantial evidence consists of information gleaned from witnesses and documents that point to an individual as the perpetrator of a crime. Physical evidence consists of actual objects ？bodies, weapons, body fluid stains, fingerprints, hairs, fibers, etc. ？that are associated with the crime and may be linked to the perpetrator.
It is the work of forensic scientists to examine the physical evidence, and using the methods of science, to reconstruct the events that constituted the crime. The prosecutor must then combine this data with statements of witnesses and evidence from documents such as correspondence, telephone records and credit card receipts to develop an overall theory of the case which can be presented in court.
Scientific evidence is an increasingly important part of both civil and criminal trials. Forensic science is a growth industry. New technologies for analyzing physical evidence are growing rapidly and private companies are becoming an increasingly important resource for the legal system. The testimony of experts is the primary means of introducing scientific evidence. Because these experts are imparting information "beyond the ken" of the layperson, they must present information that goes beyond first hand observation, opinions and hearsay not permitted under ordinary rules of evidence. Lay witnesses are constrained to testify only about matters they have directly observed. Expert witnesses are allowed to draw inferences from facts which the judge or jury is not competent to draw. They may also rely on seminars, publications, records and conversations with other experts that are part of their normal course of business.
Discovery
Despite fictional presentations to the contrary （Perry Mason is a prime example）, there are very few surprises in actual trials. This is because of the process called discovery, whereby opposing attorneys are permitted to learn the facts and expert opinions upon which the other side is basing its case prior to the actual trial. In addition, each side is required to provide the other side with a list of its witnesses before the start of trial.
Providing discovery materials in criminal cases is binding only upon the prosecution in all but a few states and Canada. Access to materials through the discovery process is the main avenue the defense has for learning what evidence will be presented against the accused at trial. This allows the defense to re-examine the evidence and develop alternative hypotheses to the prosecutor's case.
In California, the defense's access to scientific evidence is defined in the Griffin decision which provides that the defense can have the evidence only after the prosecution has completed their testing. Also, under both Griffin and a U.S. Supreme Court decision in Arizona v. Youngblood, the prosecution may consume the evidence in the testing process, as long as they act in good faith.
Beginning in 1989, furious battles erupted over discovery efforts in the DNA war. Gen-erally the defense has been able to examine autoradiographs from the case in question, laboratory reports, and the lab notes that support them in addition to the lab procedure manuals and proficiency testing results. Requests for additional materials such as other autoradiographs, validation studies, population data bases, and raw data face harsher scrutiny and often were not honored. Early DNA cases were marked by long and costly litigation over discovery. The defense claims that the prosecution and the labs they employ "stonewall' discovery requests. The lab resist discovery, maintaining that the requested materials are privileged, constitute trade secrets, are legally irrelevant.
Forensic labs also claim that the defense regularly makes overly burdensome and duplicitous requests for reams of material. If they were required to comply, lab personnel would be doing little else than identifying and duplicating discovery materials. DNA discovery battles are still being strenuously fought. Indeed, O.J. Simpson's chief lawyer, Robert Shapiro, has labeled Cellmark a "discovery outlaw." However most commentators would agree that many of the issues surrounding discovery already have been litigated or settled in other ways.
Scientific Evidence Admissibility Standards
The key element in whether scientific evidence is admissible is whether it is trustworthy. To be considered trustworthy, it must demonstrate accuracy （validity） and consistency （reliability）. Admissibility is determined by the Frye rule, which stresses "general acceptance" or by the Federal Rules of Evidence （followed by some state courts） which stress helpfulness, reliability, and relevance.
In all of the trials to date in which DNA evidence has been involved, courts have ruled it as evidence or, on appeal, have remanded the case to the trial court in 22 reported cases and have limited its admissibility in 16 cases, generally because of statistical questions.
The Frye Standard
In the 1923 decision United States v. Frye, a District of Columbia circuit court ruled against the admissibility of lie detector evidence in a murder case because the technology had not been accepted in the relevant scientific community. Since then, most state courts have followed this general standard on whether or not to allow novel scientific evidence. The so-called Frye hearing gives the prosecution and defense the opportunity to attack adverse scientific evidence and try to keep it out of the trial. The key paragraph in this decision reads：
Just when a scientific principle or discovery crosses the line between experimental and demonstratable stages is difficult to define. Somewhere in the twilight zone the evidential force of the principle must be recognized, and while courts will go a long way in admitting expert testimony deduced from well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs. Determining "general acceptance" according to the Frye standard is a two-step procedure： （1） identifying the particular field（s） into which the scientific principle or discovery falls and the relevant scientific community； and （2） determining whether that community accepts the technology, principle, or discovery. Both the underlying theory and the procedures used to produce results must be generally accepted by scientists in the relevant fields.
To these two criteria has been added a third in some jurisdictions. In California, the additional standard evolved from the 1976 decision in People v. Kelly which held that "the proponent of the evidence must demonstrate that correct scientific procedures were done in the particular case." This third "prong" also was accepted by the court in the 1989 landmark New York v. Castro, the first case in history of where DNA evidence was excluded. A distinction lost on some courts is that the Kelly rule only requires that correct procedures be used, not that the court must determine that these procedures were performed correctly.
Legal evidence rules generally hold that how well work is performed should not be the subject of an admissibility hearing because the quality of testing in a particular case goes to the weight of the evidence and not its admissibility. It is up to the trier of fact, the judge or the jury, to determine how much weight or consideration to give that evidence. In practice, the legal distinction between the admissibility of scientific testing and the weight that should be given that testing has become increasingly blurred in DNA evidenciary hearings.
It should be noted that, at least in California, the Frye rule does not require absolute "unanimity of views within the scientific community," which, according to the California Court of Appeals, would "demand the impossible." In People v. Guerra （restated in the Reilly case）, the court ruled that, "the test is met if the use of the technique is supported by a clear majority of the members of that community." Nevertheless, some courts have interpreted "general acceptance" to mean the absence of controversy, an unrealistic standard in almost any scientific or technical area.
One result of this interpretation is that Frye hearings often last longer than many trials. The mother of all Frye hearings took place in San Diego in 1987. At issue was the methodology of pre-DNA blood typing. A personal vendetta between two experts, known as the "starch wars," exacerbated the controversy which dragged on for a full year. While courts and attorneys are often reluctant to reveal the costs of proceedings, Frye hearings are expensive.
In King County （Seattle）, Washington the situation may be the worst in the country. There have been over a dozen pre-trial DNA evidentiary hearings in this jurisdiction, each requiring two to six weeks of courtroom time. Most have concerned identical technologies and laboratories. In total, these hearings are estimated to have cost the local government over $1,000,000, not counting the time that personnel such as bailiffs and guards, attorneys, experts and others have lost to more productive tasks. This might be a reasonable price to pay if the controversy had been finally settled, but at this point there is no end in sight. All costs are paid by the state in more than 90% of these cases because the defendant is indigent.
The Frye rule has been criticized for its overly conservative approach and its vulnerability to manipulation by those seeking to exclude novel scientific evidence. After the Federal Rules of Evidence were enacted, a number of jurisdictions abandoned Frye.
The Federal Standard
The Federal Rules of Evidence currently in force were promulgated by the Supreme Court and enacted by Congress in 1975. While they are applicable directly only to proceedings in federal courts, they serve as the model for evidence codes in 32 states. Despite this state recognition of federal standards, the majority of states profess to follow the Frye rule, creating evidenciary ambiguity that may not be resolved until appellate courts or legislatures address the issue.
While not explicitly repudiating the Frye rule, the Federal Rules adopt a more permissive approach. They liken the standard for scientific evidence to that for other evidence, i.e. whether the probativeness, materiality, and reliability of the evidence outweighs its tendency to mislead, prejudice, and confuse the jury. The judge has more discretion under the Federal Rules.
Rule 702, which concerns admissibility, states：
if scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise. Rule 703 requires that the facts or data presented be "of a type reasonably relied on by experts in the particular field." Rule 403 excludes evidence that would cause undue prejudice or confusion. Proponents of the Federal Rules approach to admissibility believe that taken together, these rules address all the concerns embodied in the Frye rule.
Daubert v. Merrell Dow
Critics of the Federal Rules fear that the courts may be opening themselves to "junk science" by relaxing Frye, but a landmark case heard by the Supreme Court in 1993 rejected that claim. In Daubert v. Merrell Dow Pharmaceutical, Inc., the court unanimously held that the Frye rule was incompatible with and had been superseded by the adoption of the Federal Rules. It found that "vigorous cross-examination, presentation of contrary evidence, and careful [jury] instruction are the traditional and appropriate means of attacking shaky but admissible evidence." Trial courts also could still render summary judgments and directed verdicts where there was an insufficient showing of reliability.
The effect of Daubert on states where rules mirror federal standards has yet to be felt. However, it is reasonable to assume that expert testimony on DNA will be admissible after a threshold finding that it is relevant and reliable. Defendants in these jurisdictions will have a harder time suppressing DNA evidence, although stiff challenges to its admissibility will undoubtedly continue, at least for the near future. Daubert will have little or no effect on states where Frye still prevails. Some of the states even have supreme court decisions affirming Frye. The most recent state to affirm Frye is California in the Leahy case, decided in October 1994. In these states, new high court decisions or legislation are the only means to change admissibility standards.
Legislated Admissibility
By the Fall of 1994, eleven states had statutes mandating the admissibility of DNA evidence. Maryland became the first state to do so followed by Minnesota, Louisiana, and Nevada, all in 1989. Most of the legislation contains language that DNA testing is acceptable "without antecedent expert testimony" that it is "a trustworthy and reliable method." Arguably, these laws do not cover DNA analysis methods introduced after their passage, and the defense may still challenge laboratory performance and the statistical interpretation of results. As a more sophisticated defense bar mounts increasing numbers of expensive challenges to DNA evidence, it is likely that additional state legislatures will address this issue.
Expert Witnesses
While many expert witnesses represent the best in their profession, the proliferation of expert witnesses, often considered to be "hired guns" employed to shoot holes in the other side's testimony, is a remarkable development in the criminal justice system. There is hardly any kind of case not affected by these duelling experts, but psychological, medical, and DNA testimony seems to bring out the worst of them. It is difficult not to conclude that some of these individuals are willing to stretch or ignore the facts, distort the science, and become "liars for hire." Many of these witnesses derive a substantial amount or even the bulk of their income from testifying, which should be considered in determining their credibility and weighing their testimony. One California judge bemoaned the use of such witnesses by candidly calling them the beneficiaries of "a welfare system for academics." A recent article on the ethics and responsibility of expert witnesses suggests the following criteria for qualification：
Undergraduate and graduate degrees in the relevant field of expertise；Specialized training in the subject area as it relates to forensic science；Some training in forensics；Professional licenses or certifications required by professional groups in the expert's discipline；Evidence of experimentation, teaching, and publication within the specialty area； and Prior disciplinary evidence directly relevant to the issues being considered.
Other elements that help to determine an expert's qualifications include： post-graduate training, publication in peer-reviewed scientific journals, the development of accepted tests and procedures, membership or leadership in appropriate scientific societies, and, only lastly, experience as an expert witness.
Ten years ago the Califonia case People v. Brown added criteria that has proven difficult to apply, ie., that the witness "must also be 錳mpartial,' that is, not so personally invested in establishing the acceptance of a technique that he might not be objective about disagreements within the relevant scientific community." Neither should a witness be so invested in denigrating a technique that he exaggerates the disagreement within the scientific community. Probably the best way of gaining the testimony of impartial witnesses is for courts, rather than litigants, to appoint and pay for expert witnesses. Such is the practice in many other countries. While it is unusual in the U.S, this procedure is within the power of state and federal courts. A notable example in a case involving DNA was United States v. Yee, where the magistrate called Eric Lander, a mathematician-turned-geneticist, as an expert witness to supplement the seventeen expert witnesses called by the prosecution and defense. It is reported that Judge Ito may call his own expert witnesses during the Simpson case Kelly-Frye hearing.
Defense Strategy
Defense witnesses mount various objections to DNA evidence. They no longer try to discredit the technology itself. Years ago, DNA typing achieved such wide acceptance and proven reliability that opponents now concentrate on two principal points of attack： （1） the quality and methodology of the laboratory work, including the lab's error rate, and （2） the statistical interpretation of data. The focus of the attacks on admissibility have changed over time. As one objection was knocked down, DNA opponents came up with another. The quality and relevancy of the arguments and of the experts is decreasing, having gone from population geneticists to bio-statisticians to statisticians from completely unrelated fields. What follows is a summary of the most frequently heard complaints about DNA typing and the responses to them that might be expected from forensic scientists. More technical objections, such as bandshifting and laboratory quality assurance, are addressed in Chapter Three.
Conflict of Interest
Prosecution and defense expert witnesses in DNA cases are arguably the most contentious and disparaging in the business. Several judges have remarked that Frye hearings over DNA can be extremely vicious. Among the charges and countercharges hurled back and forth is that the opposing witnesses should be disqualified from testifying because of a conflict of interest. To a certain extent, both sides are correct. The prosecution believes that defense witnesses in DNA hearings often have a vested interest in making sure that the subject stays controversial so that they can continue the lucrative practice of testifying. The defense often believes that a practicing forensic scientist has a built-in bias or predisposition toward the prosecution's side because of the close working relationship between crime labs and law enforcement. Indeed, criminalists often are police employees.
If the forensic scientists are leaders in their field, they may be subject to a further conflict of interest. If they have developed or invented techniques or tools, they may have a proprietary interest in advancing DNA testing. They may have financial holdings in DNA labs or may have received grant funding from public or private agencies. Certainly, a jury is entitled to know about all of these connections which should be fully disclosed. At the same time, courts acknowledge that, "simply because learned experts earn a living with their expertise should not prohibit the admissibility of their opinions," as the court ruled in a recent New Jersey case.
Integrity of Specimen
Opposing expert witnesses try to raise doubts about the way DNA evidence was gathered and tested claiming that contamination may have occurred. The usual argument is that the underlying procedures for forensic DNA testing were developed in laboratories where pure and known samples were used and retesting always was an option. While this is true, the argument doesn't mention the usual results of contamination, the ease with which it can be detected, and the safeguards that are in place in most forensic DNA labs.
In almost all cases, the results of using a specimen sufficiently contaminated to alter test results simply would be ruled inconclusive. One way a specimen might be contaminated is by genetic material from the technician performing the analysis or from the person gathering the evidence. Again, this could not possibly harm the accused as an exclusion would be the result of such mishandling. Finally, there is the possibility that some of the DNA drawn from the suspect could be accidentally mixed with DNA retrieved from the crime scene. Such an accident would yield false results, especially if PCR amplification is used. To avoid this possibility, crime labs that perform PCR do it in isolation and under stringent conditions that minimize the risk of contamination.
DNA analysis is undeniably better than other tests in analyzing mixed specimens and overcoming a variety of contaminants. Because of its structure and relative stability, DNA can be tested even after mixture with acids, bases, gasoline, oil, or bleach.
Error Rate
The newest objection, which is at the heart of the Simpson defense team's argument to exclude DNA evidence, is the testing lab's error rate. In a field as complicated as forensic science there are many sources of error, most of which will lead to an inconclusive or no result. A false positive or negative error rate is impossible to measure because these are such rare events. These are the types of errors caused by human error or fraud. It should be noted that most of these types of mix-ups or failures in the chain of custody would lead to a false negative result which would be work in the accused's favor. These also are more likely to occur before the evidence is received by the laboratory. There is no rate of these kinds of errors that is acceptable. Fortunately, an error resulting in a miscarriage of justice has yet to be demonstrated in forensic DNA casework, although it is perhaps inevitable that it will occur someday.
Errors intrinsic to the testing systems, such as the inability to precisely measure DNA restriction fragment lengths, are well compensated for by interpretation guidelines which take these kinds of errors into account. The series of quality control steps built into the process also provide an excellent assurance of the quality of individual and laboratory performance. In most cases these steps should lead to corrective action long before a catastrophic error has occurred.
Minimizing laboratory errors requires a quality control program such as the ones which already are in place on a voluntary basis in the forensic laboratories. Almost all forensic DNA laboratories participate in programs which include proficiency testing and confirm that a minimum level of performance has been achieved. External proficiency testing also provides an ongoing comparison of inter-laboratory measurement error. These programs, led by the American Society of Crime Laboratory Directors Laboratory Accreditation Board are rapidly gaining momentum.
In the forensic field, the final arbiters of quality are the courts where experts under examination and cross-examination submit their results to the scrutiny of the opposing experts and the judge and jury. This added level of scrutiny is necessary to ensure quality forensic work and includes review of casework, retesting, and observation of particular tests by opposing experts. Both sides need to have equal access to forensic expertise in the interests of fairness and justice.
Population Genetics Estimates
The most contentious debate in forensic DNA involves the use of statistics to estimate the rarity of a given DNA profile. This is to be expected because the extraordinary rarity of these profiles is what gives them their conclusiveness as evidence. The rarities of the genetic profiles depend on the number of genes examined （usually four or five, often more）. The frequencies of the results of each gene are multiplied to reach a combined profile frequency or the final estimate which is presented to the court.
Critics contend that among certain ethnic sub-groups, there may be arrangements of gene frequencies that differ markedly from those found in the general population. They maintain that the population base used to give frequency statistics must be drawn from the suspect's particular gene pool, i.e., if a suspect is half Vietnamese and half French, the population database used to compute the probability ra
證據(jù)的本質(zhì)
證據(jù)是從中可以推理出合理結(jié)論的任何陳述或者具體的物質(zhì)。證據(jù)是一個很廣泛的類別，可以包括任何可被五官感覺到的東西，具體包括書證、物證、雙方同意的事實、證人證詞等。刑事審判中的證據(jù)關(guān)系到犯罪的目的、動機、方法和機會等。
一般來講，證據(jù)被劃分為兩種，間接證據(jù)和實物證據(jù)。間接證據(jù)包括通過證人收集的信息以及指向某一個人為罪犯的書證。實物證據(jù)包括與犯罪有關(guān)或者與罪犯相聯(lián)系的真實的物質(zhì)，如尸體、武器、人體的液體斑點、指紋、毛發(fā)和纖維等。
法醫(yī)科學(xué)家的工作就是要檢驗實物證據(jù)，使用科學(xué)方法重建構(gòu)成犯罪的事件。然后，控方必須將這些數(shù)據(jù)與證人證言、書證如信件、電話錄音、信用卡收據(jù)等結(jié)合起來，以便能夠在法庭演示整個案件過程。
科學(xué)證據(jù)在民事和刑事審判中都占據(jù)著日益重要的地位。法醫(yī)學(xué)是一個發(fā)展中的產(chǎn)業(yè)。分析實物證據(jù)的新技術(shù)正在迅速發(fā)展，私人公司正在成為司法系統(tǒng)日益重要的資源。專家證據(jù)是引入科學(xué)證據(jù)的最初方法。因為這些專家要向外行人講授“外行人知識領(lǐng)域之外”的知識，他們就必須提出根據(jù)一般證據(jù)規(guī)則所不允許的直接觀察資料、觀點和傳聞之外的知識。外行證人被限制在僅就他們直接觀察到的事物作證。專家證人則被允許根據(jù)事實進行法官或者陪審團不能作出的推理。他們還可以根據(jù)他們的日常工作如討論會、出版物、記錄或者與其他專家的會談等作出推理。
發(fā)現(xiàn)程序
盡管可以進行相反的假定推理（Perry Mason是主要的例子），在實際的審判中幾乎不會出現(xiàn)任何驚人的發(fā)現(xiàn)。這是因為稱之為“發(fā)現(xiàn)”的程序，根據(jù)這個程序，在實際的審判前，對方律師被允許獲知另一方掌握的案件事實和專家意見。除此之外，在審判開始之前，每一方都必須提供給另一方一份證人名單。
在刑事案件中提供發(fā)現(xiàn)材料是有限的，僅就指控事項提供材料，除了美國幾個州和加拿大以外，其他地方一般都是如此。通過發(fā)現(xiàn)程序獲取資料是辯方獲得有關(guān)在法庭上控方將會呈現(xiàn)對被告不利的證據(jù)的信息的主要途徑。這一程序保證了辯方對證據(jù)的再詢問，以及研究對控方案件的替代性的假定。
在加利福尼亞州，辯方有權(quán)使用科學(xué)證據(jù)是在格里芬案的判決中詳細說明的，這個判決中說，辯方只有在控方完成鑒定后才能使用科學(xué)證據(jù)。同樣，根據(jù)格里芬案以及美國法院在Arizona v. Youngblood案中的判決，控方可以毀滅證據(jù)，只要他們秉承善意。
1989年初，就DNA之戰(zhàn)中的發(fā)現(xiàn)程序爆發(fā)了一場激戰(zhàn)。通常，除了實驗室程序手冊和熟練鑒定結(jié)果之外，辯方一直都能調(diào)查所涉案件的自動射線照相、實驗室報告以及實驗室記錄等來支持他們。辯方要求額外的資料如其他自動射線照相、確認(rèn)研究、人口資料數(shù)據(jù)和原始數(shù)據(jù)等則面臨著苛刻的詳細審查，并經(jīng)常被拒絕。早期的DNA案件在發(fā)現(xiàn)程序上都是耗時耗力的，并因此而聞名。辯方主張控方和他們雇用的實驗室都妨礙了辯方的發(fā)現(xiàn)請求。實驗室反對辯方發(fā)現(xiàn)，并堅持主張說，辯方請求發(fā)現(xiàn)的資料是受到特權(quán)保護的，構(gòu)成商業(yè)秘密，在法律上也是不相關(guān)的。
法醫(yī)實驗室還主張，辯方定期要求他們提供極度繁重的、有意欺騙的大量材料，他們難以負擔(dān)。如果他們必須要滿足辯方要求的話，實驗室的工作人員就無法從事其他的工作，只能從事鑒定和復(fù)制發(fā)現(xiàn)材料的工作了。DNA發(fā)現(xiàn)程序之戰(zhàn)仍舊繼續(xù)進行著。實際上，O.J.辛普森案件的主要律師，Robert Shapiro就賦予Cellmark以“發(fā)現(xiàn)逃犯”的稱號。然而，大多數(shù)評論人士同意圍繞著發(fā)現(xiàn)程序的許多問題已經(jīng)通過訴訟或者其他途徑解決了。
科學(xué)證據(jù)的采納標(biāo)準(zhǔn)
在是否采納科學(xué)證據(jù)的問題上的關(guān)鍵因素是科學(xué)證據(jù)是否是值得信賴的。要被認(rèn)為是值得信賴的，它必須具有精確性（有效性）和連貫性（可靠性）。是否采納可以根據(jù)Frye規(guī)則來判定的，這個規(guī)則強調(diào)“普遍接受”；或者根據(jù)聯(lián)邦證據(jù)規(guī)則（許多州法院采納此規(guī)則），這個規(guī)則強調(diào)有用性、可靠性和關(guān)聯(lián)性。
至今，在涉及DNA證據(jù)的所有審判中，法院裁決作為證據(jù)使用或者經(jīng)上訴退回原法院審理的有22個案件，因此法院實際上采納DNA證據(jù)的案件為16個，這主要是根據(jù)統(tǒng)計學(xué)統(tǒng)計出來的。
Frye標(biāo)準(zhǔn)
1923年，在United States v. Frye一案中，哥倫比亞地區(qū)巡回法院裁決否定了測謊器證據(jù)在謀殺案中的使用，因為該項技術(shù)還沒有在相關(guān)的科學(xué)界得到接受。其后，大多數(shù)州法院就是否允許采納新的科學(xué)證據(jù)方面都遵循了這一標(biāo)準(zhǔn)。所謂的Frye聽證會為控方和辯方提供了攻擊對己不利的科學(xué)證據(jù)的機會，并試圖將之控制在庭審之外。該判決的關(guān)鍵段落寫道：
正當(dāng)一個科學(xué)原理或者發(fā)現(xiàn)跨越了試驗和論證階段之間的界，它就很難定義了。在邊緣地區(qū)，這個原理的證據(jù)力必須得到驗證。當(dāng)法院認(rèn)可由公認(rèn)的科學(xué)原理或發(fā)現(xiàn)中推論出來的專家證據(jù)時，推論的來源必須是經(jīng)充分確定、并在其所屬專門領(lǐng)域獲得廣泛接受的。
根據(jù)Frye標(biāo)準(zhǔn)判定“普遍接受”是一個兩步驟的程序：（1）確定科學(xué)原理或發(fā)現(xiàn)所屬以及相關(guān)的科學(xué)界的專門領(lǐng)域；（2）判定是否該科學(xué)界已經(jīng)接受該技術(shù)、原理或發(fā)現(xiàn)。基本的理論和用于產(chǎn)生結(jié)果的程序都必須得到相關(guān)領(lǐng)域科學(xué)家的普遍接受。
在某些地區(qū)，還有第三條標(biāo)準(zhǔn)。在加利福尼亞州，第三條標(biāo)準(zhǔn)來源于1976年P(guān)eople v. Kelly一案中的判決，判決認(rèn)為“證據(jù)的提出者必須證明在特定案件中采取了正確的科學(xué)程序?！边@第三條標(biāo)準(zhǔn)在1989年法院對New York v. Castro這一劃時代案件的判決中再次得到接受，這個案件是歷排除DNA證據(jù)的第一個案件。后來的許多法院在適用這一標(biāo)準(zhǔn)時出現(xiàn)了差別，Kelly案件中的標(biāo)準(zhǔn)僅要求使用正確的程序，并沒有要求法院判定這些程序是否被正確的執(zhí)行。
法定證據(jù)規(guī)則一般認(rèn)為，程序如何更好的進行不應(yīng)該是證據(jù)可采性聽證會的主題，因為特定案件中鑒定質(zhì)量影響的是證據(jù)的重要性而不是它的可采性。應(yīng)該由審問者、法官或者陪審團來決定應(yīng)該給予證據(jù)多大的重要性。實際上，在DNA證據(jù)聽證會上，科學(xué)鑒定的可采性和重要性的法律差別已經(jīng)變得越來越模糊了。
應(yīng)該注意的是，至少在加利福尼亞州，F(xiàn)rye規(guī)則也不需要“科學(xué)界內(nèi)部觀點的完全一致”，根據(jù)加利福尼亞上訴法院的觀點，這種要求是“不可能的”。在People v. Guerra一案中（在Reilly案中重申），法院裁決道，“如果一項技術(shù)的使用被所屬領(lǐng)域的多數(shù)成員接受就滿足了鑒定的要求?！比欢?，一些法院將“普遍接受”解釋為沒有爭議，這在幾乎任何科學(xué)或技術(shù)領(lǐng)域都是不現(xiàn)實的標(biāo)準(zhǔn)。
這種解釋的一個結(jié)果是Frye聽證會經(jīng)常比許多庭審時間還要長。1987年，所有參與Frye聽證會的人士在圣地亞哥召開會議，爭議的問題是DNA鑒定之前血液類型的方法論問題。兩位專家之間的個人恩怨，稱為“淀粉之戰(zhàn)”，加劇了這場爭論，而后又延伸了一年。雖然法院和律師經(jīng)常不愿披露訴訟成本，F(xiàn)rye聽證會也是非常昂貴的。
華盛頓的King縣（西雅圖）的情況可能是這個國家中最糟糕的。在這個區(qū)域內(nèi)有超過12個審前DNA聽證會，每個都需要2－6星期的審判時間。大多數(shù)案件都涉及同樣的技術(shù)和實驗室?？傮w上，據(jù)估計這些聽證會要花費地方政府1百萬多美元，還不包括下列人員如法警、獄警、律師、專家以及其他人失去的從事其他更加有價值的工作的時間。如果爭議最終得以解決，這可能還是一個合理的價格，但是在目前這一點上，還無法確知爭議的結(jié)束。90％的案件成本是由州來支付的，因為被告都比較窮困。
因其方法上的過度保守和操作上的弱點，F(xiàn)rye規(guī)則一直受到那些尋求排除新奇科學(xué)證據(jù)的人士的批評。聯(lián)邦證據(jù)規(guī)則頒布之后，許多地區(qū)都摒棄了Frye規(guī)則。
聯(lián)邦證據(jù)規(guī)則
目前生效的聯(lián)邦證據(jù)規(guī)則是在1975年由國會制定、法院頒布的。雖然它們僅直接適用于聯(lián)邦法院的訴訟，但是它們在32個州內(nèi)都作為證據(jù)法的范本。盡管各州對聯(lián)邦標(biāo)準(zhǔn)的認(rèn)可，多數(shù)州表示要遵循Frye規(guī)則，這就造成了證據(jù)上的不確定，除非上訴法院或者立法機關(guān)解決這個問題，否則這個問題無法得到解決。
聯(lián)邦證據(jù)規(guī)則雖然沒有明確批判Frye規(guī)則，但是它們采納了一種更加寬容的方法。它們將科學(xué)證據(jù)標(biāo)準(zhǔn)與其他標(biāo)準(zhǔn)相比較，如是否證據(jù)的檢驗性、實質(zhì)性和可靠性會在價值上超過它使陪審團誤導(dǎo)、形成偏見和混淆的傾向。根據(jù)聯(lián)邦證據(jù)規(guī)則，法官有更大的自由裁量權(quán)。
涉及到證據(jù)可采性的規(guī)則702規(guī)定：
如果科學(xué)、技術(shù)、或者其他專門知識能夠幫助事實的審問者了解證據(jù)或者判定爭議中的事實，那么滿足作為專家要求的知識、技能、經(jīng)驗、訓(xùn)練或者教育的證人就可以專家意見或者其他形式作證。
規(guī)則703要求，提出的事實或者數(shù)據(jù)應(yīng)該屬于“某一專門領(lǐng)域?qū)＜液侠硇刨嚨姆N類”。規(guī)則403排除了會引起不適當(dāng)?shù)钠娀蚧煜淖C據(jù)。聯(lián)邦證據(jù)規(guī)則可采性方法的支持者相信，這些規(guī)則能夠包含所有Frye規(guī)則中包含的事項。
Daubert v. Merrell Dow
聯(lián)邦證據(jù)規(guī)則的批評者擔(dān)心法院可能因為放棄Frye規(guī)則而輕信“垃圾科學(xué)”，但是法院在1993年的一個具有里程碑意義的案件中否認(rèn)了這種主張。在Daubert v. Merrell Dow Pharmaceutical, Inc.一案中，法院全體一致的認(rèn)為，F(xiàn)rye規(guī)則與聯(lián)邦證據(jù)規(guī)則不能相容，且已經(jīng)被后者的通過而取代。法院發(fā)現(xiàn)“有力的交叉詢問、相反證據(jù)的呈遞以及謹(jǐn)慎的（陪審團）指令是傳統(tǒng)的、適當(dāng)?shù)墓舨豢煽康牡刹杉{的證據(jù)的方法。”初審法院仍可以作出簡易判決和指導(dǎo)性的陪審團裁決，但這不能充分顯示證據(jù)的可靠性。
Dauber一案對采納聯(lián)邦標(biāo)準(zhǔn)規(guī)則的州的影響還沒有發(fā)現(xiàn)。然而，可以合理假定的是，在法院裁決專家證據(jù)具有相關(guān)性和可靠性之后，就DNA問題專家證據(jù)是可以采納的。在這些管轄區(qū)域，被告將會很難排除DNA證據(jù)，雖然對它可采性的艱難挑戰(zhàn)仍舊會不容懷疑的繼續(xù)著，至少在不遠的將來如此。Dauber一案對盛行Frye規(guī)則的州來說幾乎沒有任何影響。一些州甚至用法院的判決來肯定Frye規(guī)則。1994年10月，加利福尼亞州在Leahy案中確認(rèn)了Frye規(guī)則。在這些州內(nèi)，新的高等法院或者立法機關(guān)是改變證據(jù)可采性標(biāo)準(zhǔn)的途徑。
立法上的可采性
到1994年秋季，11個州通過法令要求在法庭上采納DNA證據(jù)。馬里蘭州是第一個這樣做的州，接著是明尼蘇達州、路易斯安娜州和內(nèi)華達州，它們都是在1989年通過的。大多數(shù)法律中都包含這樣的語言，即，即使“沒有專家證據(jù)的先例”，DNA鑒定是也是可接受的；它是一個“值得信賴的、可靠的方法”。有爭議的是，這些法律都沒有包含DNA分析方法，被告仍舊可以挑戰(zhàn)實驗室的工作和結(jié)果的統(tǒng)計解釋。對于一個更加富有經(jīng)驗的被告來講，律師可能會對DNA證據(jù)發(fā)動更多極其昂貴的挑戰(zhàn)，對此，州立法機關(guān)很可能會以其他方式解決這一問題。
專家證人
一般而言，專家證人是他們所處行業(yè)中的佼佼者，專家證人經(jīng)常被認(rèn)為是用來攻擊對方證詞的“職業(yè)槍支”，專家證人的迅速擴大是刑事司法制度的一個顯著發(fā)展。幾乎沒有任何種類的案件不受這些專家的影響，但是心理學(xué)、醫(yī)學(xué)和DNA證據(jù)看起來會給他們帶來很多的麻煩。很難下結(jié)論說一些人會樂于延伸或者忽略事實、歪曲科學(xué)以及成為“說謊者”。許多專家證人收入的很大一部分都來源于作證，在判定他們證詞的可信性和重要性方面時，這應(yīng)當(dāng)作為一個因素被考慮進去。一個加利福尼亞州法官坦白地稱他們?yōu)椤皩W(xué)術(shù)福利制度”的受益者，并哀嘆專家證人的使用。最近一篇有關(guān)于專家證人的道德和責(zé)任的文章中對專家證人提出下述資格標(biāo)準(zhǔn)：
1、在相關(guān)專業(yè)領(lǐng)域具有本科和研究生學(xué)位；2、在與法醫(yī)學(xué)相關(guān)的主體領(lǐng)域經(jīng)歷過專門的訓(xùn)練；3、在辯論學(xué)方面經(jīng)受過一些訓(xùn)練；4、在專家學(xué)科內(nèi)擁有職業(yè)團體要求的職業(yè)執(zhí)照或者證明；5、在專業(yè)領(lǐng)域擁有實驗、教學(xué)和出版的證明；6、先前擁有與作證問題直接相關(guān)的學(xué)科證明。
其他幫助判定專家資格的因素包括：研究生訓(xùn)練、在同等評論性科學(xué)期刊上發(fā)表文章，對可接受的試驗和程序的發(fā)展，在適當(dāng)科學(xué)協(xié)會的成員資格或領(lǐng)導(dǎo)人員，以及最后，作為專家證人的經(jīng)驗。
10年前，加利福尼亞People v. Brown一案的判決中增加了一條事后證明是很難適用的標(biāo)準(zhǔn)，即證人“還必須是公正無私的，也就是說，專家證人不能以個人態(tài)度接受一項技術(shù)，如果在相關(guān)的科學(xué)界這項技術(shù)還存在爭論，而他的態(tài)度可能不是客觀的”。證人也不應(yīng)該毀損一項技術(shù)，夸大它在科學(xué)界的爭論。也許獲得公正無私的證人的方法是由法庭任命或者法庭支付專家證人的薪酬，而不是由當(dāng)事人。這也是許多其他國家的實踐。這種程序在美國還不是很通常，決定采用何種程序是各州和聯(lián)邦法院的權(quán)力。涉及到DNA證據(jù)的一個很的案件是United States v. Yee，Eric Lander是該案的專家證人，他原來是數(shù)學(xué)家，后來成為一名遺傳學(xué)家，它負責(zé)補充其他由控方和辯方召集的17個專家證人的證言。據(jù)報道，在辛普森案件的Kelly-Frye聽證會上，法官Ito也召集了他自己的專家證人。
辯論技巧
辯方證人對DNA證據(jù)提出了各種各樣的異議。他們不再試圖懷疑這種技術(shù)本身。幾年前，DNA鑒定贏得了廣泛的接受并證實了它的可靠性，現(xiàn)在，反對者開始集中于兩點進行攻擊：（1）實驗室工作的質(zhì)量和方法論，包括實驗室的誤差率；（2）數(shù)據(jù)的統(tǒng)計學(xué)解釋。隨著時間的流逝，對DNA證據(jù)可采性攻擊的焦點已經(jīng)轉(zhuǎn)變了。當(dāng)一個焦點被否決時，DNA證據(jù)的反對者就會推出下一個焦點。爭論和專家的質(zhì)量和關(guān)聯(lián)性都在下降，已經(jīng)從人口遺傳學(xué)家轉(zhuǎn)移到生物統(tǒng)計學(xué)家，再到完全不相關(guān)的領(lǐng)域的統(tǒng)計學(xué)家。接下來的是最頻繁聽到的對DNA鑒定的抱怨摘要以及可能來自于法醫(yī)學(xué)家對DNA鑒定的反應(yīng)。
利益沖突
在DNA案件中，控方和辯方的專家證人是這一領(lǐng)域內(nèi)爭議和漠視的。幾個法官評論說在Frye案中召開有關(guān)DNA的聽證會是極端錯誤的。在訴訟和反訴訟中，爭議的焦點是辯方證人因為利益沖突應(yīng)該被取消作證資格。在某種特定的程度上，雙方都是正確的。控方認(rèn)為，在DNA聽證會上辯方證人經(jīng)常有一個既定的目的，即確保主題保持有爭議，這樣他們才能繼續(xù)進行對他們有利的作證。而辯方經(jīng)常認(rèn)為，因為犯罪實驗室和法律實施之間工作上的密切關(guān)系，開業(yè)的法醫(yī)學(xué)家對辯方有一種固有的偏愛或傾向。實際上，刑事學(xué)家通常也是警察機關(guān)的雇員。
如果法醫(yī)學(xué)家是他們所在領(lǐng)域的，他們可能會遭遇更進一步的利益沖突。如果他們發(fā)展或者發(fā)明新技術(shù)或者工具，他們可能在發(fā)展DNA鑒定方面擁有自己私人的利益。他們可能還對DNA實驗室擁有股份，或者接受來自公共或私人機構(gòu)的資金捐助。當(dāng)然，陪審團有權(quán)了解所有的這些關(guān)系，這些關(guān)系也都應(yīng)當(dāng)被完全披露。同時，法院承認(rèn)，“不能僅因為有學(xué)問的專家可以利用他們的技能謀生，就禁止采納他們的意見。”這正如法院在最近的新澤西發(fā)生的一個案件中說的。
樣本的完整性
辯方專家證人試圖增加對DNA證據(jù)在搜集和鑒定方法上的懷疑，聲稱污染可能發(fā)生。通常的爭論焦點是法醫(yī)DNA鑒定的基礎(chǔ)程序是在實驗室中完成的，而實驗室中采用的是純凈的和已知的樣本，再次鑒定始終是一項選擇權(quán)。雖然這是真實的，但是爭論并沒有提到污染后的通常結(jié)果，被檢測到的容易，以及大多數(shù)法醫(yī)DNA實驗室的安全措施。
在幾乎所有的案件中，使用一個被充分污染的樣本以改變鑒定結(jié)果的結(jié)果將會被裁決為非決定性的。樣本可能被污染的一種途徑是來源于進行分析操作的技術(shù)人員或者搜集證據(jù)的工作人員的遺傳物質(zhì)。對此，這不可能對被告有任何損害，因為這種違反操作規(guī)程的結(jié)果會被排除。最后，存在這樣一種可能性，來源于疑犯的DNA可能偶然的與犯罪現(xiàn)場找到的DNA相混合。這樣的偶然事件會產(chǎn)生錯誤的結(jié)果，尤其是當(dāng)使用PCR擴大程序時。為了避免這種可能性，進行PCR試驗的犯罪實驗室會在隔絕的前提進行操作，在嚴(yán)格的情況下使污染的風(fēng)險降到最小。
不可否認(rèn)，在分析混和樣本和克服各種各樣的污染物方面，DNA分析比其他鑒定要好的多。因為它的結(jié)構(gòu)和相對穩(wěn)定性，甚至在與酸、堿、汽油、石油或者漂白劑混和的情況下，DNA鑒定還是可以進行。
誤差率
最新的反對理由是鑒定實驗室的誤差率，這也是辛普森案件中辯護隊排除DNA證據(jù)的論據(jù)的核心。像法醫(yī)學(xué)這樣復(fù)雜的領(lǐng)域，有許多產(chǎn)生錯誤的來源，大多數(shù)都會導(dǎo)致不確定的或者沒有結(jié)果。一個錯誤的正或負誤差率是不可能衡量的，因為這些是很罕見的事件。這些是由人為誤差或者故意導(dǎo)致的誤差類型。應(yīng)當(dāng)注意，保管鏈上大多數(shù)類型的混淆或者失誤都會導(dǎo)致假陽性的結(jié)果，這是對被告有利的。這在證據(jù)搜集到實驗室之前更可能發(fā)生。沒有任何誤差率能夠被接受。幸運的是，在DNA個案中，因為誤差導(dǎo)致誤判還沒有被發(fā)現(xiàn)，雖然不可避免的在將來的某一天會出現(xiàn)。
鑒定系統(tǒng)本身固有的誤差，如不能精確的測量DNA限制酵素片段長度，可以由解釋規(guī)則來很好的彌補，因為解釋規(guī)則會考慮到這些種類的誤差。并入該程序的一系列質(zhì)量控制步驟也為個人和實驗室的操作質(zhì)量提供了良好的保證。在大多數(shù)案件中，在致命錯誤出現(xiàn)的很早之前，這些控制步驟就應(yīng)該進行矯正性的行動了。
將實驗室誤差最小化需要一個質(zhì)量控制程序，如那些已經(jīng)在法醫(yī)實驗室主動就位的程序。幾乎所有的法醫(yī)DNA實驗室都參加了該程序，包括熟練鑒定以及確認(rèn)最低操作水平已經(jīng)被滿足的程序。外部熟練鑒定還為實驗室內(nèi)部測量誤差提供了時時比較程序。這些由美國犯罪實驗室理事實驗室鑒定合格董事會協(xié)會倡導(dǎo)的程序正在迅速傳播。
在法醫(yī)學(xué)領(lǐng)域，質(zhì)量的最后仲裁者是法庭，在法庭上，在詢問和交叉詢問下，專家證人提交他們的鑒定結(jié)果交給對方專家、法官和陪審團進行詳細審查。為了確保鑒定工作的質(zhì)量，額外的審查標(biāo)準(zhǔn)是必要的，包括個案審查、再鑒定和由對方專家就專門鑒定進行的檢查。雙方需要有權(quán)同等獲取法庭的專門技術(shù)，以滿足公平和正義的要求。
群體遺傳學(xué)評估
在法庭DNA領(lǐng)域最有爭議的辯論是使用統(tǒng)計學(xué)評估給定DNA圖譜的稀有性。這個問題實際上是被期待解決，因為正是這些圖譜的相當(dāng)稀有才賦予了它們作為證據(jù)的決定性作用。這些遺傳圖譜的稀有性要依靠被檢驗的基因數(shù)目（通常是4或5個，或者更多）。每個基因結(jié)果的頻率相乘得到一個合并的圖譜頻率或者最后呈現(xiàn)在法庭上的最后評估。
批評者主張，在特定的種族群體中，可能會出現(xiàn)基因頻率安排顯著不同于在一般人群中發(fā)現(xiàn)的安排。他們聲稱用于頻率統(tǒng)計的人口基數(shù)必須從嫌疑犯的特定基因庫中獲得。
Evidence is any statement or material object from which reasonable conclusions can be drawn. It is a broad category embracing anything perceptible to the five senses including documents, exhibits, facts agreed to by both sides, and the testimony of witnesses. Evidence in a criminal trial concerns the intent, motive, means, and opportunity to commit a crime.
In general, evidence is divided into two categories： circumstantial and physical. Circumstantial evidence consists of information gleaned from witnesses and documents that point to an individual as the perpetrator of a crime. Physical evidence consists of actual objects ？bodies, weapons, body fluid stains, fingerprints, hairs, fibers, etc. ？that are associated with the crime and may be linked to the perpetrator.
It is the work of forensic scientists to examine the physical evidence, and using the methods of science, to reconstruct the events that constituted the crime. The prosecutor must then combine this data with statements of witnesses and evidence from documents such as correspondence, telephone records and credit card receipts to develop an overall theory of the case which can be presented in court.
Scientific evidence is an increasingly important part of both civil and criminal trials. Forensic science is a growth industry. New technologies for analyzing physical evidence are growing rapidly and private companies are becoming an increasingly important resource for the legal system. The testimony of experts is the primary means of introducing scientific evidence. Because these experts are imparting information "beyond the ken" of the layperson, they must present information that goes beyond first hand observation, opinions and hearsay not permitted under ordinary rules of evidence. Lay witnesses are constrained to testify only about matters they have directly observed. Expert witnesses are allowed to draw inferences from facts which the judge or jury is not competent to draw. They may also rely on seminars, publications, records and conversations with other experts that are part of their normal course of business.
Discovery
Despite fictional presentations to the contrary （Perry Mason is a prime example）, there are very few surprises in actual trials. This is because of the process called discovery, whereby opposing attorneys are permitted to learn the facts and expert opinions upon which the other side is basing its case prior to the actual trial. In addition, each side is required to provide the other side with a list of its witnesses before the start of trial.
Providing discovery materials in criminal cases is binding only upon the prosecution in all but a few states and Canada. Access to materials through the discovery process is the main avenue the defense has for learning what evidence will be presented against the accused at trial. This allows the defense to re-examine the evidence and develop alternative hypotheses to the prosecutor's case.
In California, the defense's access to scientific evidence is defined in the Griffin decision which provides that the defense can have the evidence only after the prosecution has completed their testing. Also, under both Griffin and a U.S. Supreme Court decision in Arizona v. Youngblood, the prosecution may consume the evidence in the testing process, as long as they act in good faith.
Beginning in 1989, furious battles erupted over discovery efforts in the DNA war. Gen-erally the defense has been able to examine autoradiographs from the case in question, laboratory reports, and the lab notes that support them in addition to the lab procedure manuals and proficiency testing results. Requests for additional materials such as other autoradiographs, validation studies, population data bases, and raw data face harsher scrutiny and often were not honored. Early DNA cases were marked by long and costly litigation over discovery. The defense claims that the prosecution and the labs they employ "stonewall' discovery requests. The lab resist discovery, maintaining that the requested materials are privileged, constitute trade secrets, are legally irrelevant.
Forensic labs also claim that the defense regularly makes overly burdensome and duplicitous requests for reams of material. If they were required to comply, lab personnel would be doing little else than identifying and duplicating discovery materials. DNA discovery battles are still being strenuously fought. Indeed, O.J. Simpson's chief lawyer, Robert Shapiro, has labeled Cellmark a "discovery outlaw." However most commentators would agree that many of the issues surrounding discovery already have been litigated or settled in other ways.
Scientific Evidence Admissibility Standards
The key element in whether scientific evidence is admissible is whether it is trustworthy. To be considered trustworthy, it must demonstrate accuracy （validity） and consistency （reliability）. Admissibility is determined by the Frye rule, which stresses "general acceptance" or by the Federal Rules of Evidence （followed by some state courts） which stress helpfulness, reliability, and relevance.
In all of the trials to date in which DNA evidence has been involved, courts have ruled it as evidence or, on appeal, have remanded the case to the trial court in 22 reported cases and have limited its admissibility in 16 cases, generally because of statistical questions.
The Frye Standard
In the 1923 decision United States v. Frye, a District of Columbia circuit court ruled against the admissibility of lie detector evidence in a murder case because the technology had not been accepted in the relevant scientific community. Since then, most state courts have followed this general standard on whether or not to allow novel scientific evidence. The so-called Frye hearing gives the prosecution and defense the opportunity to attack adverse scientific evidence and try to keep it out of the trial. The key paragraph in this decision reads：
Just when a scientific principle or discovery crosses the line between experimental and demonstratable stages is difficult to define. Somewhere in the twilight zone the evidential force of the principle must be recognized, and while courts will go a long way in admitting expert testimony deduced from well-recognized scientific principle or discovery, the thing from which the deduction is made must be sufficiently established to have gained general acceptance in the particular field in which it belongs. Determining "general acceptance" according to the Frye standard is a two-step procedure： （1） identifying the particular field（s） into which the scientific principle or discovery falls and the relevant scientific community； and （2） determining whether that community accepts the technology, principle, or discovery. Both the underlying theory and the procedures used to produce results must be generally accepted by scientists in the relevant fields.
To these two criteria has been added a third in some jurisdictions. In California, the additional standard evolved from the 1976 decision in People v. Kelly which held that "the proponent of the evidence must demonstrate that correct scientific procedures were done in the particular case." This third "prong" also was accepted by the court in the 1989 landmark New York v. Castro, the first case in history of where DNA evidence was excluded. A distinction lost on some courts is that the Kelly rule only requires that correct procedures be used, not that the court must determine that these procedures were performed correctly.
Legal evidence rules generally hold that how well work is performed should not be the subject of an admissibility hearing because the quality of testing in a particular case goes to the weight of the evidence and not its admissibility. It is up to the trier of fact, the judge or the jury, to determine how much weight or consideration to give that evidence. In practice, the legal distinction between the admissibility of scientific testing and the weight that should be given that testing has become increasingly blurred in DNA evidenciary hearings.
It should be noted that, at least in California, the Frye rule does not require absolute "unanimity of views within the scientific community," which, according to the California Court of Appeals, would "demand the impossible." In People v. Guerra （restated in the Reilly case）, the court ruled that, "the test is met if the use of the technique is supported by a clear majority of the members of that community." Nevertheless, some courts have interpreted "general acceptance" to mean the absence of controversy, an unrealistic standard in almost any scientific or technical area.
One result of this interpretation is that Frye hearings often last longer than many trials. The mother of all Frye hearings took place in San Diego in 1987. At issue was the methodology of pre-DNA blood typing. A personal vendetta between two experts, known as the "starch wars," exacerbated the controversy which dragged on for a full year. While courts and attorneys are often reluctant to reveal the costs of proceedings, Frye hearings are expensive.
In King County （Seattle）, Washington the situation may be the worst in the country. There have been over a dozen pre-trial DNA evidentiary hearings in this jurisdiction, each requiring two to six weeks of courtroom time. Most have concerned identical technologies and laboratories. In total, these hearings are estimated to have cost the local government over $1,000,000, not counting the time that personnel such as bailiffs and guards, attorneys, experts and others have lost to more productive tasks. This might be a reasonable price to pay if the controversy had been finally settled, but at this point there is no end in sight. All costs are paid by the state in more than 90% of these cases because the defendant is indigent.
The Frye rule has been criticized for its overly conservative approach and its vulnerability to manipulation by those seeking to exclude novel scientific evidence. After the Federal Rules of Evidence were enacted, a number of jurisdictions abandoned Frye.
The Federal Standard
The Federal Rules of Evidence currently in force were promulgated by the Supreme Court and enacted by Congress in 1975. While they are applicable directly only to proceedings in federal courts, they serve as the model for evidence codes in 32 states. Despite this state recognition of federal standards, the majority of states profess to follow the Frye rule, creating evidenciary ambiguity that may not be resolved until appellate courts or legislatures address the issue.
While not explicitly repudiating the Frye rule, the Federal Rules adopt a more permissive approach. They liken the standard for scientific evidence to that for other evidence, i.e. whether the probativeness, materiality, and reliability of the evidence outweighs its tendency to mislead, prejudice, and confuse the jury. The judge has more discretion under the Federal Rules.
Rule 702, which concerns admissibility, states：
if scientific, technical, or other specialized knowledge will assist the trier of fact to understand the evidence or to determine a fact in issue, a witness qualified as an expert by knowledge, skill, experience, training, or education, may testify thereto in the form of an opinion or otherwise. Rule 703 requires that the facts or data presented be "of a type reasonably relied on by experts in the particular field." Rule 403 excludes evidence that would cause undue prejudice or confusion. Proponents of the Federal Rules approach to admissibility believe that taken together, these rules address all the concerns embodied in the Frye rule.
Daubert v. Merrell Dow
Critics of the Federal Rules fear that the courts may be opening themselves to "junk science" by relaxing Frye, but a landmark case heard by the Supreme Court in 1993 rejected that claim. In Daubert v. Merrell Dow Pharmaceutical, Inc., the court unanimously held that the Frye rule was incompatible with and had been superseded by the adoption of the Federal Rules. It found that "vigorous cross-examination, presentation of contrary evidence, and careful [jury] instruction are the traditional and appropriate means of attacking shaky but admissible evidence." Trial courts also could still render summary judgments and directed verdicts where there was an insufficient showing of reliability.
The effect of Daubert on states where rules mirror federal standards has yet to be felt. However, it is reasonable to assume that expert testimony on DNA will be admissible after a threshold finding that it is relevant and reliable. Defendants in these jurisdictions will have a harder time suppressing DNA evidence, although stiff challenges to its admissibility will undoubtedly continue, at least for the near future. Daubert will have little or no effect on states where Frye still prevails. Some of the states even have supreme court decisions affirming Frye. The most recent state to affirm Frye is California in the Leahy case, decided in October 1994. In these states, new high court decisions or legislation are the only means to change admissibility standards.
Legislated Admissibility
By the Fall of 1994, eleven states had statutes mandating the admissibility of DNA evidence. Maryland became the first state to do so followed by Minnesota, Louisiana, and Nevada, all in 1989. Most of the legislation contains language that DNA testing is acceptable "without antecedent expert testimony" that it is "a trustworthy and reliable method." Arguably, these laws do not cover DNA analysis methods introduced after their passage, and the defense may still challenge laboratory performance and the statistical interpretation of results. As a more sophisticated defense bar mounts increasing numbers of expensive challenges to DNA evidence, it is likely that additional state legislatures will address this issue.
Expert Witnesses
While many expert witnesses represent the best in their profession, the proliferation of expert witnesses, often considered to be "hired guns" employed to shoot holes in the other side's testimony, is a remarkable development in the criminal justice system. There is hardly any kind of case not affected by these duelling experts, but psychological, medical, and DNA testimony seems to bring out the worst of them. It is difficult not to conclude that some of these individuals are willing to stretch or ignore the facts, distort the science, and become "liars for hire." Many of these witnesses derive a substantial amount or even the bulk of their income from testifying, which should be considered in determining their credibility and weighing their testimony. One California judge bemoaned the use of such witnesses by candidly calling them the beneficiaries of "a welfare system for academics." A recent article on the ethics and responsibility of expert witnesses suggests the following criteria for qualification：
Undergraduate and graduate degrees in the relevant field of expertise；Specialized training in the subject area as it relates to forensic science；Some training in forensics；Professional licenses or certifications required by professional groups in the expert's discipline；Evidence of experimentation, teaching, and publication within the specialty area； and Prior disciplinary evidence directly relevant to the issues being considered.
Other elements that help to determine an expert's qualifications include： post-graduate training, publication in peer-reviewed scientific journals, the development of accepted tests and procedures, membership or leadership in appropriate scientific societies, and, only lastly, experience as an expert witness.
Ten years ago the Califonia case People v. Brown added criteria that has proven difficult to apply, ie., that the witness "must also be 錳mpartial,' that is, not so personally invested in establishing the acceptance of a technique that he might not be objective about disagreements within the relevant scientific community." Neither should a witness be so invested in denigrating a technique that he exaggerates the disagreement within the scientific community. Probably the best way of gaining the testimony of impartial witnesses is for courts, rather than litigants, to appoint and pay for expert witnesses. Such is the practice in many other countries. While it is unusual in the U.S, this procedure is within the power of state and federal courts. A notable example in a case involving DNA was United States v. Yee, where the magistrate called Eric Lander, a mathematician-turned-geneticist, as an expert witness to supplement the seventeen expert witnesses called by the prosecution and defense. It is reported that Judge Ito may call his own expert witnesses during the Simpson case Kelly-Frye hearing.
Defense Strategy
Defense witnesses mount various objections to DNA evidence. They no longer try to discredit the technology itself. Years ago, DNA typing achieved such wide acceptance and proven reliability that opponents now concentrate on two principal points of attack： （1） the quality and methodology of the laboratory work, including the lab's error rate, and （2） the statistical interpretation of data. The focus of the attacks on admissibility have changed over time. As one objection was knocked down, DNA opponents came up with another. The quality and relevancy of the arguments and of the experts is decreasing, having gone from population geneticists to bio-statisticians to statisticians from completely unrelated fields. What follows is a summary of the most frequently heard complaints about DNA typing and the responses to them that might be expected from forensic scientists. More technical objections, such as bandshifting and laboratory quality assurance, are addressed in Chapter Three.
Conflict of Interest
Prosecution and defense expert witnesses in DNA cases are arguably the most contentious and disparaging in the business. Several judges have remarked that Frye hearings over DNA can be extremely vicious. Among the charges and countercharges hurled back and forth is that the opposing witnesses should be disqualified from testifying because of a conflict of interest. To a certain extent, both sides are correct. The prosecution believes that defense witnesses in DNA hearings often have a vested interest in making sure that the subject stays controversial so that they can continue the lucrative practice of testifying. The defense often believes that a practicing forensic scientist has a built-in bias or predisposition toward the prosecution's side because of the close working relationship between crime labs and law enforcement. Indeed, criminalists often are police employees.
If the forensic scientists are leaders in their field, they may be subject to a further conflict of interest. If they have developed or invented techniques or tools, they may have a proprietary interest in advancing DNA testing. They may have financial holdings in DNA labs or may have received grant funding from public or private agencies. Certainly, a jury is entitled to know about all of these connections which should be fully disclosed. At the same time, courts acknowledge that, "simply because learned experts earn a living with their expertise should not prohibit the admissibility of their opinions," as the court ruled in a recent New Jersey case.
Integrity of Specimen
Opposing expert witnesses try to raise doubts about the way DNA evidence was gathered and tested claiming that contamination may have occurred. The usual argument is that the underlying procedures for forensic DNA testing were developed in laboratories where pure and known samples were used and retesting always was an option. While this is true, the argument doesn't mention the usual results of contamination, the ease with which it can be detected, and the safeguards that are in place in most forensic DNA labs.
In almost all cases, the results of using a specimen sufficiently contaminated to alter test results simply would be ruled inconclusive. One way a specimen might be contaminated is by genetic material from the technician performing the analysis or from the person gathering the evidence. Again, this could not possibly harm the accused as an exclusion would be the result of such mishandling. Finally, there is the possibility that some of the DNA drawn from the suspect could be accidentally mixed with DNA retrieved from the crime scene. Such an accident would yield false results, especially if PCR amplification is used. To avoid this possibility, crime labs that perform PCR do it in isolation and under stringent conditions that minimize the risk of contamination.
DNA analysis is undeniably better than other tests in analyzing mixed specimens and overcoming a variety of contaminants. Because of its structure and relative stability, DNA can be tested even after mixture with acids, bases, gasoline, oil, or bleach.
Error Rate
The newest objection, which is at the heart of the Simpson defense team's argument to exclude DNA evidence, is the testing lab's error rate. In a field as complicated as forensic science there are many sources of error, most of which will lead to an inconclusive or no result. A false positive or negative error rate is impossible to measure because these are such rare events. These are the types of errors caused by human error or fraud. It should be noted that most of these types of mix-ups or failures in the chain of custody would lead to a false negative result which would be work in the accused's favor. These also are more likely to occur before the evidence is received by the laboratory. There is no rate of these kinds of errors that is acceptable. Fortunately, an error resulting in a miscarriage of justice has yet to be demonstrated in forensic DNA casework, although it is perhaps inevitable that it will occur someday.
Errors intrinsic to the testing systems, such as the inability to precisely measure DNA restriction fragment lengths, are well compensated for by interpretation guidelines which take these kinds of errors into account. The series of quality control steps built into the process also provide an excellent assurance of the quality of individual and laboratory performance. In most cases these steps should lead to corrective action long before a catastrophic error has occurred.
Minimizing laboratory errors requires a quality control program such as the ones which already are in place on a voluntary basis in the forensic laboratories. Almost all forensic DNA laboratories participate in programs which include proficiency testing and confirm that a minimum level of performance has been achieved. External proficiency testing also provides an ongoing comparison of inter-laboratory measurement error. These programs, led by the American Society of Crime Laboratory Directors Laboratory Accreditation Board are rapidly gaining momentum.
In the forensic field, the final arbiters of quality are the courts where experts under examination and cross-examination submit their results to the scrutiny of the opposing experts and the judge and jury. This added level of scrutiny is necessary to ensure quality forensic work and includes review of casework, retesting, and observation of particular tests by opposing experts. Both sides need to have equal access to forensic expertise in the interests of fairness and justice.
Population Genetics Estimates
The most contentious debate in forensic DNA involves the use of statistics to estimate the rarity of a given DNA profile. This is to be expected because the extraordinary rarity of these profiles is what gives them their conclusiveness as evidence. The rarities of the genetic profiles depend on the number of genes examined （usually four or five, often more）. The frequencies of the results of each gene are multiplied to reach a combined profile frequency or the final estimate which is presented to the court.
Critics contend that among certain ethnic sub-groups, there may be arrangements of gene frequencies that differ markedly from those found in the general population. They maintain that the population base used to give frequency statistics must be drawn from the suspect's particular gene pool, i.e., if a suspect is half Vietnamese and half French, the population database used to compute the probability ra
證據(jù)的本質(zhì)
證據(jù)是從中可以推理出合理結(jié)論的任何陳述或者具體的物質(zhì)。證據(jù)是一個很廣泛的類別，可以包括任何可被五官感覺到的東西，具體包括書證、物證、雙方同意的事實、證人證詞等。刑事審判中的證據(jù)關(guān)系到犯罪的目的、動機、方法和機會等。
一般來講，證據(jù)被劃分為兩種，間接證據(jù)和實物證據(jù)。間接證據(jù)包括通過證人收集的信息以及指向某一個人為罪犯的書證。實物證據(jù)包括與犯罪有關(guān)或者與罪犯相聯(lián)系的真實的物質(zhì)，如尸體、武器、人體的液體斑點、指紋、毛發(fā)和纖維等。
法醫(yī)科學(xué)家的工作就是要檢驗實物證據(jù)，使用科學(xué)方法重建構(gòu)成犯罪的事件。然后，控方必須將這些數(shù)據(jù)與證人證言、書證如信件、電話錄音、信用卡收據(jù)等結(jié)合起來，以便能夠在法庭演示整個案件過程。
科學(xué)證據(jù)在民事和刑事審判中都占據(jù)著日益重要的地位。法醫(yī)學(xué)是一個發(fā)展中的產(chǎn)業(yè)。分析實物證據(jù)的新技術(shù)正在迅速發(fā)展，私人公司正在成為司法系統(tǒng)日益重要的資源。專家證據(jù)是引入科學(xué)證據(jù)的最初方法。因為這些專家要向外行人講授“外行人知識領(lǐng)域之外”的知識，他們就必須提出根據(jù)一般證據(jù)規(guī)則所不允許的直接觀察資料、觀點和傳聞之外的知識。外行證人被限制在僅就他們直接觀察到的事物作證。專家證人則被允許根據(jù)事實進行法官或者陪審團不能作出的推理。他們還可以根據(jù)他們的日常工作如討論會、出版物、記錄或者與其他專家的會談等作出推理。
發(fā)現(xiàn)程序
盡管可以進行相反的假定推理（Perry Mason是主要的例子），在實際的審判中幾乎不會出現(xiàn)任何驚人的發(fā)現(xiàn)。這是因為稱之為“發(fā)現(xiàn)”的程序，根據(jù)這個程序，在實際的審判前，對方律師被允許獲知另一方掌握的案件事實和專家意見。除此之外，在審判開始之前，每一方都必須提供給另一方一份證人名單。
在刑事案件中提供發(fā)現(xiàn)材料是有限的，僅就指控事項提供材料，除了美國幾個州和加拿大以外，其他地方一般都是如此。通過發(fā)現(xiàn)程序獲取資料是辯方獲得有關(guān)在法庭上控方將會呈現(xiàn)對被告不利的證據(jù)的信息的主要途徑。這一程序保證了辯方對證據(jù)的再詢問，以及研究對控方案件的替代性的假定。
在加利福尼亞州，辯方有權(quán)使用科學(xué)證據(jù)是在格里芬案的判決中詳細說明的，這個判決中說，辯方只有在控方完成鑒定后才能使用科學(xué)證據(jù)。同樣，根據(jù)格里芬案以及美國法院在Arizona v. Youngblood案中的判決，控方可以毀滅證據(jù)，只要他們秉承善意。
1989年初，就DNA之戰(zhàn)中的發(fā)現(xiàn)程序爆發(fā)了一場激戰(zhàn)。通常，除了實驗室程序手冊和熟練鑒定結(jié)果之外，辯方一直都能調(diào)查所涉案件的自動射線照相、實驗室報告以及實驗室記錄等來支持他們。辯方要求額外的資料如其他自動射線照相、確認(rèn)研究、人口資料數(shù)據(jù)和原始數(shù)據(jù)等則面臨著苛刻的詳細審查，并經(jīng)常被拒絕。早期的DNA案件在發(fā)現(xiàn)程序上都是耗時耗力的，并因此而聞名。辯方主張控方和他們雇用的實驗室都妨礙了辯方的發(fā)現(xiàn)請求。實驗室反對辯方發(fā)現(xiàn)，并堅持主張說，辯方請求發(fā)現(xiàn)的資料是受到特權(quán)保護的，構(gòu)成商業(yè)秘密，在法律上也是不相關(guān)的。
法醫(yī)實驗室還主張，辯方定期要求他們提供極度繁重的、有意欺騙的大量材料，他們難以負擔(dān)。如果他們必須要滿足辯方要求的話，實驗室的工作人員就無法從事其他的工作，只能從事鑒定和復(fù)制發(fā)現(xiàn)材料的工作了。DNA發(fā)現(xiàn)程序之戰(zhàn)仍舊繼續(xù)進行著。實際上，O.J.辛普森案件的主要律師，Robert Shapiro就賦予Cellmark以“發(fā)現(xiàn)逃犯”的稱號。然而，大多數(shù)評論人士同意圍繞著發(fā)現(xiàn)程序的許多問題已經(jīng)通過訴訟或者其他途徑解決了。
科學(xué)證據(jù)的采納標(biāo)準(zhǔn)
在是否采納科學(xué)證據(jù)的問題上的關(guān)鍵因素是科學(xué)證據(jù)是否是值得信賴的。要被認(rèn)為是值得信賴的，它必須具有精確性（有效性）和連貫性（可靠性）。是否采納可以根據(jù)Frye規(guī)則來判定的，這個規(guī)則強調(diào)“普遍接受”；或者根據(jù)聯(lián)邦證據(jù)規(guī)則（許多州法院采納此規(guī)則），這個規(guī)則強調(diào)有用性、可靠性和關(guān)聯(lián)性。
至今，在涉及DNA證據(jù)的所有審判中，法院裁決作為證據(jù)使用或者經(jīng)上訴退回原法院審理的有22個案件，因此法院實際上采納DNA證據(jù)的案件為16個，這主要是根據(jù)統(tǒng)計學(xué)統(tǒng)計出來的。
Frye標(biāo)準(zhǔn)
1923年，在United States v. Frye一案中，哥倫比亞地區(qū)巡回法院裁決否定了測謊器證據(jù)在謀殺案中的使用，因為該項技術(shù)還沒有在相關(guān)的科學(xué)界得到接受。其后，大多數(shù)州法院就是否允許采納新的科學(xué)證據(jù)方面都遵循了這一標(biāo)準(zhǔn)。所謂的Frye聽證會為控方和辯方提供了攻擊對己不利的科學(xué)證據(jù)的機會，并試圖將之控制在庭審之外。該判決的關(guān)鍵段落寫道：
正當(dāng)一個科學(xué)原理或者發(fā)現(xiàn)跨越了試驗和論證階段之間的界，它就很難定義了。在邊緣地區(qū)，這個原理的證據(jù)力必須得到驗證。當(dāng)法院認(rèn)可由公認(rèn)的科學(xué)原理或發(fā)現(xiàn)中推論出來的專家證據(jù)時，推論的來源必須是經(jīng)充分確定、并在其所屬專門領(lǐng)域獲得廣泛接受的。
根據(jù)Frye標(biāo)準(zhǔn)判定“普遍接受”是一個兩步驟的程序：（1）確定科學(xué)原理或發(fā)現(xiàn)所屬以及相關(guān)的科學(xué)界的專門領(lǐng)域；（2）判定是否該科學(xué)界已經(jīng)接受該技術(shù)、原理或發(fā)現(xiàn)。基本的理論和用于產(chǎn)生結(jié)果的程序都必須得到相關(guān)領(lǐng)域科學(xué)家的普遍接受。
在某些地區(qū)，還有第三條標(biāo)準(zhǔn)。在加利福尼亞州，第三條標(biāo)準(zhǔn)來源于1976年P(guān)eople v. Kelly一案中的判決，判決認(rèn)為“證據(jù)的提出者必須證明在特定案件中采取了正確的科學(xué)程序?！边@第三條標(biāo)準(zhǔn)在1989年法院對New York v. Castro這一劃時代案件的判決中再次得到接受，這個案件是歷排除DNA證據(jù)的第一個案件。后來的許多法院在適用這一標(biāo)準(zhǔn)時出現(xiàn)了差別，Kelly案件中的標(biāo)準(zhǔn)僅要求使用正確的程序，并沒有要求法院判定這些程序是否被正確的執(zhí)行。
法定證據(jù)規(guī)則一般認(rèn)為，程序如何更好的進行不應(yīng)該是證據(jù)可采性聽證會的主題，因為特定案件中鑒定質(zhì)量影響的是證據(jù)的重要性而不是它的可采性。應(yīng)該由審問者、法官或者陪審團來決定應(yīng)該給予證據(jù)多大的重要性。實際上，在DNA證據(jù)聽證會上，科學(xué)鑒定的可采性和重要性的法律差別已經(jīng)變得越來越模糊了。
應(yīng)該注意的是，至少在加利福尼亞州，F(xiàn)rye規(guī)則也不需要“科學(xué)界內(nèi)部觀點的完全一致”，根據(jù)加利福尼亞上訴法院的觀點，這種要求是“不可能的”。在People v. Guerra一案中（在Reilly案中重申），法院裁決道，“如果一項技術(shù)的使用被所屬領(lǐng)域的多數(shù)成員接受就滿足了鑒定的要求?！比欢?，一些法院將“普遍接受”解釋為沒有爭議，這在幾乎任何科學(xué)或技術(shù)領(lǐng)域都是不現(xiàn)實的標(biāo)準(zhǔn)。
這種解釋的一個結(jié)果是Frye聽證會經(jīng)常比許多庭審時間還要長。1987年，所有參與Frye聽證會的人士在圣地亞哥召開會議，爭議的問題是DNA鑒定之前血液類型的方法論問題。兩位專家之間的個人恩怨，稱為“淀粉之戰(zhàn)”，加劇了這場爭論，而后又延伸了一年。雖然法院和律師經(jīng)常不愿披露訴訟成本，F(xiàn)rye聽證會也是非常昂貴的。
華盛頓的King縣（西雅圖）的情況可能是這個國家中最糟糕的。在這個區(qū)域內(nèi)有超過12個審前DNA聽證會，每個都需要2－6星期的審判時間。大多數(shù)案件都涉及同樣的技術(shù)和實驗室?？傮w上，據(jù)估計這些聽證會要花費地方政府1百萬多美元，還不包括下列人員如法警、獄警、律師、專家以及其他人失去的從事其他更加有價值的工作的時間。如果爭議最終得以解決，這可能還是一個合理的價格，但是在目前這一點上，還無法確知爭議的結(jié)束。90％的案件成本是由州來支付的，因為被告都比較窮困。
因其方法上的過度保守和操作上的弱點，F(xiàn)rye規(guī)則一直受到那些尋求排除新奇科學(xué)證據(jù)的人士的批評。聯(lián)邦證據(jù)規(guī)則頒布之后，許多地區(qū)都摒棄了Frye規(guī)則。
聯(lián)邦證據(jù)規(guī)則
目前生效的聯(lián)邦證據(jù)規(guī)則是在1975年由國會制定、法院頒布的。雖然它們僅直接適用于聯(lián)邦法院的訴訟，但是它們在32個州內(nèi)都作為證據(jù)法的范本。盡管各州對聯(lián)邦標(biāo)準(zhǔn)的認(rèn)可，多數(shù)州表示要遵循Frye規(guī)則，這就造成了證據(jù)上的不確定，除非上訴法院或者立法機關(guān)解決這個問題，否則這個問題無法得到解決。
聯(lián)邦證據(jù)規(guī)則雖然沒有明確批判Frye規(guī)則，但是它們采納了一種更加寬容的方法。它們將科學(xué)證據(jù)標(biāo)準(zhǔn)與其他標(biāo)準(zhǔn)相比較，如是否證據(jù)的檢驗性、實質(zhì)性和可靠性會在價值上超過它使陪審團誤導(dǎo)、形成偏見和混淆的傾向。根據(jù)聯(lián)邦證據(jù)規(guī)則，法官有更大的自由裁量權(quán)。
涉及到證據(jù)可采性的規(guī)則702規(guī)定：
如果科學(xué)、技術(shù)、或者其他專門知識能夠幫助事實的審問者了解證據(jù)或者判定爭議中的事實，那么滿足作為專家要求的知識、技能、經(jīng)驗、訓(xùn)練或者教育的證人就可以專家意見或者其他形式作證。
規(guī)則703要求，提出的事實或者數(shù)據(jù)應(yīng)該屬于“某一專門領(lǐng)域?qū)＜液侠硇刨嚨姆N類”。規(guī)則403排除了會引起不適當(dāng)?shù)钠娀蚧煜淖C據(jù)。聯(lián)邦證據(jù)規(guī)則可采性方法的支持者相信，這些規(guī)則能夠包含所有Frye規(guī)則中包含的事項。
Daubert v. Merrell Dow
聯(lián)邦證據(jù)規(guī)則的批評者擔(dān)心法院可能因為放棄Frye規(guī)則而輕信“垃圾科學(xué)”，但是法院在1993年的一個具有里程碑意義的案件中否認(rèn)了這種主張。在Daubert v. Merrell Dow Pharmaceutical, Inc.一案中，法院全體一致的認(rèn)為，F(xiàn)rye規(guī)則與聯(lián)邦證據(jù)規(guī)則不能相容，且已經(jīng)被后者的通過而取代。法院發(fā)現(xiàn)“有力的交叉詢問、相反證據(jù)的呈遞以及謹(jǐn)慎的（陪審團）指令是傳統(tǒng)的、適當(dāng)?shù)墓舨豢煽康牡刹杉{的證據(jù)的方法。”初審法院仍可以作出簡易判決和指導(dǎo)性的陪審團裁決，但這不能充分顯示證據(jù)的可靠性。
Dauber一案對采納聯(lián)邦標(biāo)準(zhǔn)規(guī)則的州的影響還沒有發(fā)現(xiàn)。然而，可以合理假定的是，在法院裁決專家證據(jù)具有相關(guān)性和可靠性之后，就DNA問題專家證據(jù)是可以采納的。在這些管轄區(qū)域，被告將會很難排除DNA證據(jù)，雖然對它可采性的艱難挑戰(zhàn)仍舊會不容懷疑的繼續(xù)著，至少在不遠的將來如此。Dauber一案對盛行Frye規(guī)則的州來說幾乎沒有任何影響。一些州甚至用法院的判決來肯定Frye規(guī)則。1994年10月，加利福尼亞州在Leahy案中確認(rèn)了Frye規(guī)則。在這些州內(nèi)，新的高等法院或者立法機關(guān)是改變證據(jù)可采性標(biāo)準(zhǔn)的途徑。
立法上的可采性
到1994年秋季，11個州通過法令要求在法庭上采納DNA證據(jù)。馬里蘭州是第一個這樣做的州，接著是明尼蘇達州、路易斯安娜州和內(nèi)華達州，它們都是在1989年通過的。大多數(shù)法律中都包含這樣的語言，即，即使“沒有專家證據(jù)的先例”，DNA鑒定是也是可接受的；它是一個“值得信賴的、可靠的方法”。有爭議的是，這些法律都沒有包含DNA分析方法，被告仍舊可以挑戰(zhàn)實驗室的工作和結(jié)果的統(tǒng)計解釋。對于一個更加富有經(jīng)驗的被告來講，律師可能會對DNA證據(jù)發(fā)動更多極其昂貴的挑戰(zhàn)，對此，州立法機關(guān)很可能會以其他方式解決這一問題。
專家證人
一般而言，專家證人是他們所處行業(yè)中的佼佼者，專家證人經(jīng)常被認(rèn)為是用來攻擊對方證詞的“職業(yè)槍支”，專家證人的迅速擴大是刑事司法制度的一個顯著發(fā)展。幾乎沒有任何種類的案件不受這些專家的影響，但是心理學(xué)、醫(yī)學(xué)和DNA證據(jù)看起來會給他們帶來很多的麻煩。很難下結(jié)論說一些人會樂于延伸或者忽略事實、歪曲科學(xué)以及成為“說謊者”。許多專家證人收入的很大一部分都來源于作證，在判定他們證詞的可信性和重要性方面時，這應(yīng)當(dāng)作為一個因素被考慮進去。一個加利福尼亞州法官坦白地稱他們?yōu)椤皩W(xué)術(shù)福利制度”的受益者，并哀嘆專家證人的使用。最近一篇有關(guān)于專家證人的道德和責(zé)任的文章中對專家證人提出下述資格標(biāo)準(zhǔn)：
1、在相關(guān)專業(yè)領(lǐng)域具有本科和研究生學(xué)位；2、在與法醫(yī)學(xué)相關(guān)的主體領(lǐng)域經(jīng)歷過專門的訓(xùn)練；3、在辯論學(xué)方面經(jīng)受過一些訓(xùn)練；4、在專家學(xué)科內(nèi)擁有職業(yè)團體要求的職業(yè)執(zhí)照或者證明；5、在專業(yè)領(lǐng)域擁有實驗、教學(xué)和出版的證明；6、先前擁有與作證問題直接相關(guān)的學(xué)科證明。
其他幫助判定專家資格的因素包括：研究生訓(xùn)練、在同等評論性科學(xué)期刊上發(fā)表文章，對可接受的試驗和程序的發(fā)展，在適當(dāng)科學(xué)協(xié)會的成員資格或領(lǐng)導(dǎo)人員，以及最后，作為專家證人的經(jīng)驗。
10年前，加利福尼亞People v. Brown一案的判決中增加了一條事后證明是很難適用的標(biāo)準(zhǔn)，即證人“還必須是公正無私的，也就是說，專家證人不能以個人態(tài)度接受一項技術(shù)，如果在相關(guān)的科學(xué)界這項技術(shù)還存在爭論，而他的態(tài)度可能不是客觀的”。證人也不應(yīng)該毀損一項技術(shù)，夸大它在科學(xué)界的爭論。也許獲得公正無私的證人的方法是由法庭任命或者法庭支付專家證人的薪酬，而不是由當(dāng)事人。這也是許多其他國家的實踐。這種程序在美國還不是很通常，決定采用何種程序是各州和聯(lián)邦法院的權(quán)力。涉及到DNA證據(jù)的一個很的案件是United States v. Yee，Eric Lander是該案的專家證人，他原來是數(shù)學(xué)家，后來成為一名遺傳學(xué)家，它負責(zé)補充其他由控方和辯方召集的17個專家證人的證言。據(jù)報道，在辛普森案件的Kelly-Frye聽證會上，法官Ito也召集了他自己的專家證人。
辯論技巧
辯方證人對DNA證據(jù)提出了各種各樣的異議。他們不再試圖懷疑這種技術(shù)本身。幾年前，DNA鑒定贏得了廣泛的接受并證實了它的可靠性，現(xiàn)在，反對者開始集中于兩點進行攻擊：（1）實驗室工作的質(zhì)量和方法論，包括實驗室的誤差率；（2）數(shù)據(jù)的統(tǒng)計學(xué)解釋。隨著時間的流逝，對DNA證據(jù)可采性攻擊的焦點已經(jīng)轉(zhuǎn)變了。當(dāng)一個焦點被否決時，DNA證據(jù)的反對者就會推出下一個焦點。爭論和專家的質(zhì)量和關(guān)聯(lián)性都在下降，已經(jīng)從人口遺傳學(xué)家轉(zhuǎn)移到生物統(tǒng)計學(xué)家，再到完全不相關(guān)的領(lǐng)域的統(tǒng)計學(xué)家。接下來的是最頻繁聽到的對DNA鑒定的抱怨摘要以及可能來自于法醫(yī)學(xué)家對DNA鑒定的反應(yīng)。
利益沖突
在DNA案件中，控方和辯方的專家證人是這一領(lǐng)域內(nèi)爭議和漠視的。幾個法官評論說在Frye案中召開有關(guān)DNA的聽證會是極端錯誤的。在訴訟和反訴訟中，爭議的焦點是辯方證人因為利益沖突應(yīng)該被取消作證資格。在某種特定的程度上，雙方都是正確的。控方認(rèn)為，在DNA聽證會上辯方證人經(jīng)常有一個既定的目的，即確保主題保持有爭議，這樣他們才能繼續(xù)進行對他們有利的作證。而辯方經(jīng)常認(rèn)為，因為犯罪實驗室和法律實施之間工作上的密切關(guān)系，開業(yè)的法醫(yī)學(xué)家對辯方有一種固有的偏愛或傾向。實際上，刑事學(xué)家通常也是警察機關(guān)的雇員。
如果法醫(yī)學(xué)家是他們所在領(lǐng)域的，他們可能會遭遇更進一步的利益沖突。如果他們發(fā)展或者發(fā)明新技術(shù)或者工具，他們可能在發(fā)展DNA鑒定方面擁有自己私人的利益。他們可能還對DNA實驗室擁有股份，或者接受來自公共或私人機構(gòu)的資金捐助。當(dāng)然，陪審團有權(quán)了解所有的這些關(guān)系，這些關(guān)系也都應(yīng)當(dāng)被完全披露。同時，法院承認(rèn)，“不能僅因為有學(xué)問的專家可以利用他們的技能謀生，就禁止采納他們的意見。”這正如法院在最近的新澤西發(fā)生的一個案件中說的。
樣本的完整性
辯方專家證人試圖增加對DNA證據(jù)在搜集和鑒定方法上的懷疑，聲稱污染可能發(fā)生。通常的爭論焦點是法醫(yī)DNA鑒定的基礎(chǔ)程序是在實驗室中完成的，而實驗室中采用的是純凈的和已知的樣本，再次鑒定始終是一項選擇權(quán)。雖然這是真實的，但是爭論并沒有提到污染后的通常結(jié)果，被檢測到的容易，以及大多數(shù)法醫(yī)DNA實驗室的安全措施。
在幾乎所有的案件中，使用一個被充分污染的樣本以改變鑒定結(jié)果的結(jié)果將會被裁決為非決定性的。樣本可能被污染的一種途徑是來源于進行分析操作的技術(shù)人員或者搜集證據(jù)的工作人員的遺傳物質(zhì)。對此，這不可能對被告有任何損害，因為這種違反操作規(guī)程的結(jié)果會被排除。最后，存在這樣一種可能性，來源于疑犯的DNA可能偶然的與犯罪現(xiàn)場找到的DNA相混合。這樣的偶然事件會產(chǎn)生錯誤的結(jié)果，尤其是當(dāng)使用PCR擴大程序時。為了避免這種可能性，進行PCR試驗的犯罪實驗室會在隔絕的前提進行操作，在嚴(yán)格的情況下使污染的風(fēng)險降到最小。
不可否認(rèn)，在分析混和樣本和克服各種各樣的污染物方面，DNA分析比其他鑒定要好的多。因為它的結(jié)構(gòu)和相對穩(wěn)定性，甚至在與酸、堿、汽油、石油或者漂白劑混和的情況下，DNA鑒定還是可以進行。
誤差率
最新的反對理由是鑒定實驗室的誤差率，這也是辛普森案件中辯護隊排除DNA證據(jù)的論據(jù)的核心。像法醫(yī)學(xué)這樣復(fù)雜的領(lǐng)域，有許多產(chǎn)生錯誤的來源，大多數(shù)都會導(dǎo)致不確定的或者沒有結(jié)果。一個錯誤的正或負誤差率是不可能衡量的，因為這些是很罕見的事件。這些是由人為誤差或者故意導(dǎo)致的誤差類型。應(yīng)當(dāng)注意，保管鏈上大多數(shù)類型的混淆或者失誤都會導(dǎo)致假陽性的結(jié)果，這是對被告有利的。這在證據(jù)搜集到實驗室之前更可能發(fā)生。沒有任何誤差率能夠被接受。幸運的是，在DNA個案中，因為誤差導(dǎo)致誤判還沒有被發(fā)現(xiàn)，雖然不可避免的在將來的某一天會出現(xiàn)。
鑒定系統(tǒng)本身固有的誤差，如不能精確的測量DNA限制酵素片段長度，可以由解釋規(guī)則來很好的彌補，因為解釋規(guī)則會考慮到這些種類的誤差。并入該程序的一系列質(zhì)量控制步驟也為個人和實驗室的操作質(zhì)量提供了良好的保證。在大多數(shù)案件中，在致命錯誤出現(xiàn)的很早之前，這些控制步驟就應(yīng)該進行矯正性的行動了。
將實驗室誤差最小化需要一個質(zhì)量控制程序，如那些已經(jīng)在法醫(yī)實驗室主動就位的程序。幾乎所有的法醫(yī)DNA實驗室都參加了該程序，包括熟練鑒定以及確認(rèn)最低操作水平已經(jīng)被滿足的程序。外部熟練鑒定還為實驗室內(nèi)部測量誤差提供了時時比較程序。這些由美國犯罪實驗室理事實驗室鑒定合格董事會協(xié)會倡導(dǎo)的程序正在迅速傳播。
在法醫(yī)學(xué)領(lǐng)域，質(zhì)量的最后仲裁者是法庭，在法庭上，在詢問和交叉詢問下，專家證人提交他們的鑒定結(jié)果交給對方專家、法官和陪審團進行詳細審查。為了確保鑒定工作的質(zhì)量，額外的審查標(biāo)準(zhǔn)是必要的，包括個案審查、再鑒定和由對方專家就專門鑒定進行的檢查。雙方需要有權(quán)同等獲取法庭的專門技術(shù)，以滿足公平和正義的要求。
群體遺傳學(xué)評估
在法庭DNA領(lǐng)域最有爭議的辯論是使用統(tǒng)計學(xué)評估給定DNA圖譜的稀有性。這個問題實際上是被期待解決，因為正是這些圖譜的相當(dāng)稀有才賦予了它們作為證據(jù)的決定性作用。這些遺傳圖譜的稀有性要依靠被檢驗的基因數(shù)目（通常是4或5個，或者更多）。每個基因結(jié)果的頻率相乘得到一個合并的圖譜頻率或者最后呈現(xiàn)在法庭上的最后評估。
批評者主張，在特定的種族群體中，可能會出現(xiàn)基因頻率安排顯著不同于在一般人群中發(fā)現(xiàn)的安排。他們聲稱用于頻率統(tǒng)計的人口基數(shù)必須從嫌疑犯的特定基因庫中獲得。