Sources of Law for the Forensic Scientist

A constitution is the foundational document of a state's legal order. Every other source of law must conform to it. For the forensic scientist, the constitutionally relevant provisions are those that protect individual rights against state interference during investigation and prosecution: protections against unreasonable search and seizure, against self-incrimination, and against deprivation of life or personal liberty without due process.

In India, Article 20(3) of the Constitution provides that no person accused of an offence shall be compelled to be a witness against themselves. Indian courts have interpreted this as protecting against testimonial compulsion, meaning forced confessions or self-incriminating statements. The Supreme Court of India held in Selvi v. State of Karnataka (2010) that techniques such as narcoanalysis, brain mapping, and polygraph tests conducted without informed consent violate Article 20(3) and Article 21. Article 21 guarantees the right to life and personal liberty and has been interpreted to encompass a right to privacy, bodily integrity, and fair procedure. Any forensic procedure that infringes Article 21 must be authorised by a law that is itself reasonable, fair, and just.

In the United States, the Fourth Amendment prohibits unreasonable searches and seizures and requires warrants to be supported by probable cause. Evidence obtained in violation of the Fourth Amendment is generally excluded under the exclusionary rule. The Fifth Amendment protects against self-incrimination and has been read by the US Supreme Court as covering compelled production of testimonial evidence. Physical evidence such as DNA or blood samples taken under statutory authority are generally treated as non-testimonial and thus outside Fifth Amendment protection, though Fourth Amendment warrant requirements still apply. In England and Wales, the Police and Criminal Evidence Act 1984 (PACE) and its Codes of Practice set the statutory framework for how police may search persons and premises and obtain samples; breach of a PACE Code may lead to exclusion of evidence under Section 78 of the Act, which gives courts discretion to exclude evidence that would have an adverse effect on the fairness of proceedings.

Below the constitution in the hierarchy sits primary legislation: statutes enacted by the national legislature. For forensic scientists, statutes are the most immediately operational source of law. They define which evidence is admissible, what procedural steps must be followed to preserve admissibility, and what penalties attach to tampering with evidence or giving false expert testimony.

India provides a clear illustration of legislative change affecting forensic practice. The Indian Evidence Act 1872, which governed admissibility for over 150 years, was replaced by the Bharatiya Sakshya Adhiniyam 2023 (BSA), which came into force on 1 July 2024. The Bharatiya Nagarik Suraksha Sanhita 2023 (BNSS) simultaneously replaced the Code of Criminal Procedure 1973. The BSA retains the broad framework for expert evidence (experts whose opinions are relevant are competent witnesses) but updates provisions on electronic records and their admissibility, which directly affects digital forensics. Forensic scientists working in India must now cite the BSA section number rather than the corresponding Indian Evidence Act provision in their reports.

Other statutes that directly affect forensic science practice include specialised laws that create powers to collect specific categories of evidence. In India, the Narcotic Drugs and Psychotropic Substances Act 1985 creates specific procedures for sampling and chemical analysis of seized substances. The Arms Act 1959 governs the seizure and examination of firearms and ammunition. The Information Technology Act 2000 (as amended) creates the framework for seizing and examining electronic evidence. Each of these imposes procedural requirements, and non-compliance may render the forensic evidence inadmissible even if the analysis itself is sound.

In England and Wales, the Criminal Procedure and Investigations Act 1996 governs disclosure obligations, including the disclosure of unused forensic material that might assist the defence. In the United States, the Federal Rules of Evidence (particularly Rules 702-706) set the standards for expert opinion evidence in federal courts, requiring that expert testimony rest on sufficient facts or data, be the product of reliable principles and methods, and reflect a reliable application of those principles to the facts. State courts follow analogous rules, though with some variation.

A statute can only go so far in specifying technical procedures. Parliament is not equipped to write laboratory protocols. It therefore delegates rule-making authority to executive bodies, government departments, or regulatory agencies, who then issue delegated legislation: statutory instruments, rules, notifications, and orders. These bind the same parties as the parent statute, but only to the extent the parent statute authorises them. Delegated legislation that purports to authorise something the parent statute does not permit, or that contradicts the parent statute, is ultra vires and void.

For forensic scientists, delegated legislation is the source that most directly governs day-to-day laboratory operations. In England and Wales, the Forensic Science Regulator was placed on a statutory footing by the Forensic Science Regulator Act 2021. The Regulator issues Codes of Practice and Conduct that set minimum quality standards for forensic science providers. While the Codes are not themselves statutory instruments, compliance with them is required for accreditation, and a court may take non-compliance into account when assessing the reliability of forensic evidence. The Home Office issues circulars and guidelines on specific forensic procedures, such as DNA sampling and the national DNA database, which supplement the primary legislation.

In India, government notifications under the parent statutes establish procedures for specific forensic disciplines. The DNA Technology (Use and Application) Regulation Bill, which has undergone multiple rounds of consultation, would, if enacted, create a delegated-legislation regime for DNA profiling. In the meantime, procedures are governed by internal laboratory rules and constitutional and statutory constraints. Internationally, ISO/IEC 17025:2017 (General Requirements for the Competence of Testing and Calibration Laboratories) functions as a de facto standard that national accreditation bodies enforce. Though ISO standards are not law, accreditation bodies are often empowered by statute, making compliance indirectly mandatory.

In common-law systems, the decisions of superior courts are themselves a source of law. The doctrine of stare decisis requires lower courts to follow the ratio decidendi (the legal reasoning that decided the case) of higher courts in the same hierarchy. For forensic scientists, this means that a ruling by the Supreme Court of India or the UK Supreme Court on the admissibility of a forensic method, or on the procedural requirements for collecting a particular type of evidence, binds every court below it. A forensic scientist who uses a method that has been held unreliable by a superior court in their jurisdiction faces a near-certain admissibility challenge.

The distinction between binding and persuasive precedent is important when forensic scientists read judgments from other jurisdictions. The US Supreme Court's decision in Daubert v. Merrell Dow Pharmaceuticals (1993), which set the federal standard for expert evidence, is not binding in India or England, but it has been cited in Indian High Court judgments and has influenced academic and judicial discussion of expert evidence standards across the common-law world. Similarly, the UK Supreme Court's decisions on forensic evidence handling under PACE have been cited in Commonwealth courts as persuasive authority.

In civil-law systems, the relationship between courts and legislation is different. Judicial decisions are not formally binding sources of law; the primary source is the codified statute. Courts interpret the code, and the interpretation by a supreme court carries great practical weight, but a subsequent court is not formally bound in the way that stare decisis requires. This means that in civil-law systems, the forensic scientist's primary reference for the legality of a procedure is the relevant code or statute, not a line of case law. In practice, the decisions of civil-law supreme courts are followed consistently enough that the difference from stare decisis is narrower than the formal doctrine suggests.

Procedural codes govern how evidence is gathered, disclosed, and presented in court. In India, the Bharatiya Nagarik Suraksha Sanhita 2023 replaced the Code of Criminal Procedure 1973 and sets out the procedural requirements for police investigation, arrest, search, seizure, and trial. Sections of the BNSS that govern the examination of material objects, the chain of custody for physical exhibits, and the procedure for sending material to a government forensic laboratory create binding obligations on both investigators and scientists. A forensic scientist who departs from these procedures creates a ground for challenging the evidence.

In England and Wales, the Criminal Procedure Rules (CrimPR) and the Criminal Practice Directions set out the procedural framework for all criminal proceedings. Part 19 of the CrimPR specifically governs expert evidence: it requires experts to state the substance of all material instructions they received, to declare any conflict of interest, to acknowledge a duty to the court that overrides the duty to the party that instructed them, and to serve a written report in advance of trial. These procedural rules have the force of law, and failure to comply can result in the expert's evidence being excluded or its weight substantially reduced.

The procedural framework differs structurally between adversarial and inquisitorial systems. In adversarial common-law systems, each party appoints its own expert, and those experts give evidence in examination and cross-examination. The forensic scientist's primary obligation is to the court (not to the instructing party), but the mechanism for testing their evidence is adversarial. In inquisitorial civil-law systems, the court appoints a single expert or a panel of experts to investigate and report. The expert's role is to advise the court neutrally, and there is no cross-examination in the common-law sense. For a forensic scientist working across jurisdictions, this distinction changes how reports are structured, what questions are in scope, and how the scientist interacts with lawyers.

At the base of the hierarchy sit professional codes and standards. These are issued by professional bodies, accreditation organisations, or regulatory authorities, and they set the ethical and technical floor for forensic science practice. Increasingly, these codes are enforced not just by professional sanctions but through court scrutiny of whether the scientist's methodology met the applicable standard. In England and Wales, the Forensic Science Regulator's Codes of Practice set mandatory quality standards for accredited providers, and courts have cited non-compliance as a factor in assessing the weight of expert evidence.

Professional codes also address conflicts of interest, confidentiality, record-keeping, and the scope of the expert's competence. A forensic scientist who gives evidence outside their area of competence, fails to maintain adequate records, or allows commercial pressure from the instructing party to distort their conclusions is in breach of professional standards regardless of whether their evidence is technically accurate. The connection between relevance and admissibility and professional compliance is direct: courts in multiple jurisdictions have excluded or reduced the weight of forensic evidence where the scientist failed to follow the applicable professional code.

When sources of law conflict, three resolution rules apply. First, a higher source overrides a lower: the constitution overrides a statute, a statute overrides delegated legislation, and statute overrides case law where Parliament has expressly reversed a judicial decision. Second, a later source overrides an earlier source at the same level (lex posterior derogat priori), provided there is an irreconcilable conflict: a statute enacted in 2024 overrides an inconsistent statute from 2005. Third, a specific source overrides a general source at the same level (lex specialis derogat generali): the Narcotic Drugs and Psychotropic Substances Act's specific requirements for sampling narcotics override general evidence-handling provisions to the extent of any conflict. Forensic scientists should be alert to all three rules, particularly when a specialist statutory regime governs their evidence and the general procedural code says something different.