There are criminal cases that cannot be resolved with testimony or documents. Determining whether injuries are consistent with an accident, calculating the loss from a fraud, analysing a substance, or establishing the authorship of a document requires specialised knowledge that goes beyond legal expertise. For this, the process provides expert evidence.
Expert evidence brings into the proceedings the technical, scientific, or artistic knowledge needed to assess certain facts. It is provided by experts: professionals who give an expert opinion. Unlike a witness, who testifies about facts they observed, the expert issues a technical judgement based on their specialisation. The Spanish system admits expert evidence proposed by the prosecution, by the defence, and appointed ex officio by the judge; the fact that both parties can propose an expert guarantees the adversarial principle in technical matters.
The most common specialities are varied. Forensic medical expertise is involved in injuries, homicides, sexual offences, and accidents. Psychological expertise assesses the defendant’s criminal responsibility, the credibility of testimonies, and psychological harm. Accounting and economic expertise is central in economic crimes, where it quantifies the loss. Computer and digital expertise analyses devices and verifies the authenticity of communications and the chain of custody. Handwriting and document expertise determines the authorship or falsity of documents. And accident-reconstruction expertise reconstructs the dynamics of traffic accidents.
Proposing one’s own expert should not be done automatically: it has costs and may be unnecessary if the weaknesses of the opposing report can be pointed out in cross-examination. But it is advisable when the prosecution’s expert evidence is the central element of the case, when it uses questionable methodologies or claims more than its specialisation allows, when there are alternative interpretations favourable to the defendant, when the quantification of the loss is decisive for the sentence, or when one wishes to establish circumstances modifying liability of a technical nature.
Expert evidence must be proposed at the right moment, mainly in the defence brief, identifying the expert and justifying the relevance of their report. The lawyer must work closely with the expert, provide the documentation, and review the report before submitting it. The choice of expert is crucial: they must have accredited specialisation, experience in judicial reports, and the ability to explain themselves clearly before the court.
When the prosecution presents an expert report, the defence may cross-examine the expert at trial. An effective cross-examination challenges the methodology used, points out the limitations of the analysis, examines the expert’s specific qualification, and reveals contradictions with other evidence or with the scientific literature.
The court is not bound to follow the expert’s conclusions: it assesses expert evidence with freedom of judgement, subject to logic, experience, and scientific knowledge, and must give reasons if it departs from it. When there are two contradictory reports, it must explain which it finds more convincing and why. If both are equally well-founded and the contradiction is genuine, that disparity can generate a reasonable doubt which, by in dubio pro reo, must be resolved in the defendant’s favour. That is why a well-conducted defence expert report can be decisive.