There are criminal cases that are not resolved with testimonies or documents. Determining whether injuries are compatible 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 has expert evidence.
Expert evidence brings into the process 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 witnessed, the expert issues a technical judgment based on their specialisation. The Spanish system admits expert evidence proposed by the prosecution, by the defence and that appointed ex officio by the judge; the fact that both parties can propose an expert guarantees the principle of confrontation in technical matters.
The most frequent specialities are varied. Forensic-medical expertise intervenes in injuries, homicides, sexual offences and accidents. Psychological expertise assesses the criminal responsibility of the accused, the credibility of testimonies and psychological harm. Accounting and economic expertise is central in economic offences, 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 incidents.
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 expertise 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 accused, when the quantification of the loss is decisive for the penalty, or when one wishes to establish circumstances modifying responsibility that are technical in nature.
Expert evidence must be proposed at the appropriate 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 them with the documentation and review the report before presenting it. The choice of expert is crucial: they must have established 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 questions 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 obliged to follow the expert's conclusions: it assesses the expertise with freedom of judgment, 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 may generate a reasonable doubt which, under in dubio pro reo, must be resolved in favour of the accused. That is why a well-conducted defence expertise can be decisive.
