The Adversarial System and the Scientist's Place in It

The adversarial model rests on the idea that the best way to find the truth is to have each side put the strongest possible version of their case, with the opposing side testing it. The judge and jury are passive: they hear what is presented and decide, but they do not go looking for evidence, do not call their own witnesses, and do not question parties independently. Each side selects its own witnesses, including its own experts, and is responsible for preparing and presenting its case. The adversarial tradition arose in English common law and spread to the countries that adopted common-law systems.

The inquisitorial model does not mean the state simply wins. Defence rights are protected by different mechanisms: the right to challenge the court-appointed expert's report, the right to submit questions for the expert, and in some systems the right to appoint a party expert in addition to the court expert. In France, a court-appointed expert (expert judiciaire) files a written report that all parties can contest before the judge. In Germany, the Sachverstaendiger appointed by the court is cross-examinable. The key structural difference is that the expert's primary loyalty runs to the court from the outset, which removes the structural incentive for partisan opinion.

Most real-world systems are hybrids. India, formally adversarial under the Bharatiya Nagarik Suraksha Sanhita 2023 and the Bharatiya Sakshya Adhiniyam 2023, gives courts broad power to ask questions of witnesses at any time and to order further investigation. Japan reformed its code of criminal procedure in 2009 to introduce lay judge panels alongside professional judges, blending inquisitorial evidence-gathering with a form of jury deliberation. The labels adversarial and inquisitorial describe tendencies, not absolute categories.

The prosecution represents the state. In England and Wales this is the Crown Prosecution Service; in the United States it is the District Attorney or federal prosecutor; in India it is the public prosecutor under the Bharatiya Nagarik Suraksha Sanhita 2023. The prosecution carries the burden of proof and must present evidence sufficient to convince the factfinder beyond reasonable doubt. The prosecution typically calls the forensic experts whose findings support guilt.

The defence represents the accused. The defence has no obligation to present any evidence at all; it can simply test the prosecution's case. In practice, the defence often calls its own expert witnesses to challenge prosecution forensic evidence, offer alternative interpretations, or contest the methodology used. The defence expert is in the same structural position as the prosecution expert: hired by one side, but owing a duty to the court.

The judge manages the trial: rules on admissibility of evidence, directs the jury on the law, and ensures fair process. The judge does not find facts in a jury trial. In a bench trial (no jury), the judge is both the legal referee and the factfinder. The jury, where present, hears the evidence and decides the facts, including the credibility and weight of expert evidence. Jurors are not required to accept expert opinions and may reject them if they have good reason, though in practice they rarely disregard uncontested expert findings.

The expert witness is not an advocate. The expert's function is to assist the court to understand technical or scientific matters that fall outside ordinary knowledge. An expert is permitted to give opinions, whereas ordinary witnesses are confined to facts they observed. But that permission to give opinion is conditional on the opinion being within the expert's genuine expertise, being based on a proper and disclosed methodology, and being presented honestly to the court regardless of which party it helps.

The burden of proof in a criminal trial sits on the prosecution throughout. The defendant is presumed innocent and is not required to prove anything. This means that if the evidence is in equipoise, neither clearly establishing guilt nor clearly establishing innocence, the verdict must be not guilty. The prosecution must affirmatively discharge its burden.

The standard of proof in criminal proceedings is beyond reasonable doubt. In England and Wales, the direction to a jury is that they must be sure of guilt. In the United States, the phrase 'beyond a reasonable doubt' is used directly. Under the Bharatiya Sakshya Adhiniyam 2023, Indian courts apply the same standard inherited from common law. 'Sure' and 'beyond reasonable doubt' are treated as equivalent: the jury or judge must have no doubt based on reason. A 95 percent probability is not enough if the remaining five percent represents a rational possibility. This high standard reflects the asymmetric cost of errors: wrongful conviction is treated as worse than wrongful acquittal.

For the forensic scientist, the standards of proof have a direct implication for how conclusions are expressed. An opinion phrased as 'consistent with' is not the same as 'could only have come from'. A DNA profile match expressed as a likelihood ratio of one million to one is powerful but does not by itself establish guilt beyond reasonable doubt, because guilt requires more than a biological match: it requires the court to be sure the sample was deposited in the circumstances alleged. The expert's job is to convey what the scientific evidence shows; the job of proving guilt to the required standard belongs to the prosecution as a whole.

The structural tension in adversarial expert evidence is straightforward. The expert is selected by one party, briefed by that party's lawyers on what the case involves, paid by that party, and may be asked to appear only if their opinion is favourable. This creates an obvious selection bias before any testimony is given, and an ongoing pressure on the expert to maintain a position that helps the party that retained them. The result, at its worst, is the 'hired gun': an expert who functions as an advocate in a white coat.

The law's response in adversarial jurisdictions is to impose an overriding duty to the court that supersedes the contractual obligation to the instructing party. In England and Wales, Criminal Procedure Rules Part 19 states that an expert's duty is to help the court to achieve the overriding objective, and that duty overrides any obligation to the person from whom the expert receives instructions or by whom the expert is paid. This is not optional: an expert who suppresses evidence, overstates certainty, or withholds an opinion that would help the other side is in breach of the rule and may face sanctions, including having their evidence excluded or being reported to their professional body.

The clearest articulation of what the duty requires in practice came from the English case of The Ikarian Reefer (1993), a civil matter, but one whose principles have been adopted in criminal proceedings and in jurisdictions beyond England and Wales. The court in that case set out duties including: expert evidence must be, and be seen to be, the independent product of the expert; the expert must not assume the role of an advocate; the expert must state the facts or assumptions on which the opinion is based; the expert must not omit to consider material facts that detract from the concluded opinion; the expert should make it clear when a question falls outside their expertise; and if the expert changes their view after seeing the opposing expert's report, they must communicate that change to both sides and the court without delay.

Expert evidence is given in three stages. First, examination-in-chief (direct examination in the United States): the party that called the expert asks them questions designed to bring out their qualifications, methodology, findings, and conclusions in a structured way. The expert presents their opinion in their own words, typically referring to their written report. The examining advocate cannot ask leading questions (questions that suggest the answer) of their own witness in examination-in-chief.

Second, cross-examination: the opposing party's advocate questions the expert. Cross-examination is the primary testing mechanism of adversarial procedure. The cross-examiner may ask leading questions. Common cross-examination techniques against forensic experts include: challenging the expert's qualifications or experience in the specific area; challenging the reliability or validity of the methodology used; putting to the expert alternative scenarios consistent with the physical evidence; drawing attention to literature that contradicts the expert's approach; questioning the completeness of the examination or the handling of materials; and using the expert's own prior publications against them if those publications are inconsistent with the position taken in court. The expert's obligation is to answer honestly, to concede what is fair to concede, and to maintain positions that are scientifically defensible without yielding to advocacy pressure.

Third, re-examination: the party that called the expert may ask further questions, but only on matters that arose in cross-examination. Re-examination cannot introduce new topics. Its purpose is to clarify or correct any impression left by cross-examination that the expert considers misleading. The expert cannot use re-examination to simply repeat their evidence-in-chief.

After all parties have examined the expert, the judge may ask questions. In a bench trial the judge's questions can be extensive. In jury trials in England and Wales, individual jurors cannot question witnesses, but in some jurisdictions, including some US states, jurors may submit written questions for the judge to put to the expert. See Surviving Cross-Examination for preparation strategy and techniques.

Because the hired-gun problem is structural, the adversarial system has developed several institutional responses beyond the general duty to the court. None eliminates the problem entirely, but each reduces its worst effects.

• Codes of conduct and court rules. Criminal Procedure Rules Part 19 in England and Wales requires the expert to include in their report a statement that they understand their duty to the court and have complied with it. Courts in the United States increasingly require experts to comply with standards such as those of the relevant professional body (American Academy of Forensic Sciences, etc.). India's courts may assess credibility against the expert's professional standing. Breach of a code can result in evidence being excluded, costs being awarded, or referral to a professional regulator.
• Pre-trial expert meetings (without prejudice). In England and Wales, Criminal Procedure Rules Part 19 also empowers the court to direct opposing experts to meet before trial and produce a joint statement identifying the points on which they agree, the points on which they disagree, and the reasons for any disagreement. This narrows the contested issues, prevents experts from maintaining artificially extreme positions, and alerts both parties and the court to where the real scientific dispute lies.
• Single joint expert (SJE). Both parties instruct a single expert jointly, and that expert's report is submitted to the court. The SJE is more common in civil proceedings (where the Civil Procedure Rules in England and Wales encourage it for lower-value matters) but is used in some criminal contexts for discrete technical questions. The SJE reduces costs and removes the structural incentive to be partisan, but may not be appropriate where the scientific dispute is itself the central issue in the case.
• Court-appointed experts. In inquisitorial systems this is the default. In adversarial systems it is available but rarely used: both Federal Rule of Evidence 706 in the United States and the court's inherent jurisdiction in England and Wales permit the court to appoint its own independent expert. Court-appointed experts are insulated from party pressure and typically carry significant weight with the factfinder.
• Disclosure obligations. The expert must disclose the material they relied upon, the methodology they used, and, in England and Wales, any earlier draft reports or changes in opinion. Disclosure rules prevent experts from concealing data that undermines their conclusion and allow the opposing party to assess the foundations of the opinion properly. See The Expert Report for the report requirements in detail.

No single safeguard is sufficient. The combination of a clear duty to the court, professional codes of conduct, mandatory disclosure, and pre-trial expert discussion collectively narrows the space in which partisan behaviour can operate without detection. The forensic scientist who internalises the duty to the court and applies it independently of instruction, long before the safeguards are triggered, is the most reliable safeguard of all.