Facts Bearing on the Opinion of Experts: Section 40

What section 40 makes possible in practice

Putting a published paper to an expert. A defence advocate cross-examining a fingerprint examiner may put to him the published literature on erroneous identifications, or the report of a proficiency test in which examiners of his laboratory performed badly. Those are facts not otherwise relevant.

Proving a control experiment. That the same test on a known sample gave a different result, or that the reagent reacts to a substance other than blood.

Proving the error rate of a technique. This is the most valuable use and it is underused in India. A discipline that cannot state how often it is wrong is asking to be believed rather than tested.

Proving other cases. That the same examiner, on the same instrument, reported a different conclusion on materially identical facts.

Supporting the expert. The section runs both ways, so the prosecution may prove validation studies, accreditation, and the results of blind trials.

The limit that is often missed

Section 40 makes the fact relevant. It does not dispense with proving the fact. A journal article does not prove itself. Somebody must depose to it, and in practice it is put to the expert in cross-examination and he either accepts it or does not. If he does not, the party wishing to rely on it must prove it by evidence.

This is a real limitation in Indian practice, and it partly explains why scientific literature plays a far smaller part in Indian criminal trials than in some other systems.

A worked example: attacking a serology report

A murder trial in Kolhapur. The State Forensic Science Laboratory reports that the stain on the accused's shirt is human blood of group B, the deceased's group.

The direct attack is on the chain of custody and is not section 40 material at all: when was the shirt seized, what seal, when did it reach the laboratory.

The section 40 attack is on the science, and the advocate has four routes, each of which produces a fact not otherwise relevant.

One. That the presumptive test used, a benzidine or Kastle-Meyer reagent, gives a positive result with certain plant peroxidases and with some metal salts. That fact is about chemistry in general and has nothing to do with this shirt, and section 40 lets it in because it is inconsistent with the opinion.

Two. That grouping from a dried stain of that age and size is unreliable, evidenced by published studies, put to the analyst.

Three. That group B is carried by roughly a third of the population of the district, so the finding excludes nobody. The frequency of a blood group in a population is a fact not otherwise relevant to whether this accused killed this deceased.

Four. That the laboratory's own record shows the same analyst reported an inconclusive result on a comparable stain a week earlier.

And the answer in re-examination is also section 40 material: that the confirmatory test used was specific, that the control samples ran correctly, and that the analyst's laboratory is accredited and passed a proficiency trial that year.

Nothing in that exchange is about the accused, and all of it decides how much the report is worth. That is the whole point of the section.

Distinctions

What beginners get wrong

Thinking section 40 is about corroboration of the expert. It is about testing him, in either direction.

Thinking the section proves the fact. It only makes the fact relevant. It still has to be proved, usually through the mouth of the expert in cross-examination.

Ignoring it. It is a single sentence and it is skipped in most courses, but it is the only statutory doorway through which the reliability of a forensic technique can be examined at all, and a question on the weight of expert evidence is answered better by a candidate who names it.

Quick revision

• Section 40, Bharatiya Sakshya Adhiniyam, 2023, formerly section 46 of the Indian Evidence Act 1872.
• Facts not otherwise relevant become relevant if they support or are inconsistent with a relevant expert opinion.
• Parasitic: it operates only where an opinion is already relevant under section 39.
• Two illustrations: the symptoms of others poisoned by the same poison, and other harbours without sea walls, which is a control group.
• It is the doorway for published literature, validation studies, error rates, control experiments and population frequencies.
• It makes the fact relevant; it does not prove it.

Test yourself

1. State section 40 and explain what is meant by calling it parasitic. Facts not otherwise relevant are relevant if they support or are inconsistent with the opinions of experts, when such opinions are relevant. It is parasitic because it operates only where an opinion has already been made relevant, principally by section 39; if the opinion is inadmissible, section 40 has nothing to attach to.

2. Explain illustration (b) and say what scientific idea it embodies. The question is whether an obstruction to a harbour was caused by a sea wall, and the fact that other similarly situated harbours without sea walls silted up at the same time is relevant. It is the idea of a control group: if the effect appears without the supposed cause, the causal claim is weakened.

3. Give three kinds of material a defence advocate can bring in through section 40. Published literature on the error rate or limitations of the technique, the results of control experiments or proficiency tests, and the frequency of the characteristic in the relevant population.

4. What is the practical limitation of section 40 in an Indian trial? It makes the fact relevant but does not prove it. Unless the expert accepts the proposition in cross-examination, the party must prove the material by evidence, which is difficult, and this is why scientific literature features less in Indian trials than in some other systems.