Expert Witness

1. Definition, Concept, and Legal Nature

The expert witness qualifies as an auxiliary of justice (adlatere judicis), according to the wording of Article 149 of the Code of Civil Procedure (CPC/2015). From a doctrinal perspective, the expert is a trusted professional of the court, legally qualified, who has the duty to examine persons, objects, or places in order to extract technical conclusions necessary for the resolution of the dispute.

The legal nature of the expert is that of an occasional public agent, exercising a múnus publicum (public duty). Although they do not maintain a permanent statutory link with the State (except for official State experts in criminal matters), the judicial expert acts by delegation of the Judiciary. Their function is of a technical-assistential nature, possessing no decision-making power, since the magistrate is not bound by the expert report (principle of free reasoned conviction or the system of rational persuasion), as prescribed by Art. 479 of the CPC and Art. 182 of the Code of Criminal Procedure (CPP).

2. Historical Evolution and Comparative Law

Historically, the figure of the expert dates back to Roman Law, where the judex resorted to artis periti for issues requiring specific knowledge (such as land surveying or medicine). However, it was with the advent of rationalism and legal positivism that expertise was consolidated as an autonomous means of proof, moving away from ordeals and purely intuitive judgments.

In Comparative Law, there is a distinction between the Civil Law system (such as the Brazilian and French systems), where the expert is predominantly an auxiliary of the court appointed by the magistrate, and the Common Law system (Anglo-Saxon), where the figure of the expert witness prevails, presented by the parties and subjected to rigorous cross-examination. The Brazilian legislative evolution, culminating in the CPC/2015, sought a balance, strengthening the adversarial principle through the figure of the technical assistant and the possibility of consensual expertise (Art. 471, CPC).

3. Legal Provision and Normative Structure

The legal foundation of the expert institution is multifaceted, depending on the sphere of action:

• Civil Procedure: Articles 156 to 158 establish the general rules regarding the expert, while Articles 464 to 480 regulate the production of expert evidence. Art. 156, §1º, requires that experts be registered in a database maintained by the court (CPTEC).
• Criminal Procedure: Art. 158 and following of the CPP deal with the examination of the corpus delicti and expert examinations in general. Law 13.964/2019 (Anti-Crime Package) introduced crucial provisions on the chain of custody (Arts. 158-A to 158-F), reinforcing the procedural rigor required of the official expert.
• Labor Law: The CLT, in Art. 195, establishes the mandatory nature of expertise for the characterization of unhealthy or hazardous work, with the expert being mandatorily a physician or a labor engineer.
• Federal Constitution: Art. 5, item LVI, by prohibiting evidence obtained by illicit means, indirectly grounds the need for technical expertise that ensures the integrity of the evidence.

4. Practical Application and Jurisprudential Understanding

The current jurisprudence of the Superior Courts has focused on the technical qualification and impartiality of the expert. The Superior Court of Justice (STJ), through Precedent 197, has already consolidated understandings regarding the setting of expert fees, and more recently, has ruled on the lack of need for expertise when the fact can be proven by other means (AgInt in AREsp 1640523/SP).

In the scope of the Supreme Federal Court (STF), the debate on the expert gains prominence in the Chain of Custody. The STF has annulled evidence when there is a breach of isolation or improper manipulation by experts or public agents, harming the reliability of the evidence (HC 160.662). In the Superior Labor Court (TST), the central discussion revolves around the responsibility for the payment of expert fees after the Labor Reform (Law 13.467/2017), as decided in ADI 5766, which declared the unconstitutionality of provisions that forced the beneficiary of free justice to bear such costs.

5. Correlated Principles and Doctrinal Divergences

The expert's performance is governed by fundamental principles:

• Principle of Specialization: The expert must possess proven technical knowledge in the specific area of the diligence.
• Principle of Adversarial Proceedings: The parties have the right to appoint technical assistants and formulate questions (Art. 465, §1º, CPC).
• Principle of Impartiality: The expert is subject to the same causes of impediment and suspicion applied to judges (Art. 148, II, CPC).

A relevant doctrinal divergence lies in the distinction between the "official expert" and the "appointed expert." While in civil proceedings the appointment is discretionary by the judge within a registry, in criminal proceedings, priority is given to the official expert (civil servant). The discussion intensifies regarding the validity of reports signed by only one non-official expert, where the STF, through Precedent 361, requires the participation of two non-official experts in the absence of an official expert, although the CPP was amended by Law 11.690/2008 to admit only one official expert.

6. Contemporary Relevance and Practical Impacts

In contemporary times, expertise has evolved into the digital and biotechnological fields. The forensic computing expert and the geneticist expert have become central figures. The practical impact of expertise is decisive: in cases of medical malpractice, complex bankruptcies, or cybercrimes, the expert report is often the "backbone" of the sentence.

The introduction of artificial intelligence software in expert analysis raises new debates about the auditability of the algorithm and the expert's duty to explain the methodology used, preventing the report from becoming a technological "black box," which would violate the duty of substantiation provided for in Art. 473, §1º of the CPC.