Expert evidence in modern slavery and trafficking cases operates across two distinct legal contexts: the National Referral Mechanism (NRM) and criminal proceedings where a defendant claims to have been compelled to commit offences as a result of trafficking. This guide explains the legal framework, the types of expert evidence available, and how to instruct effectively.
The Modern Slavery Act 2015 consolidated and extended the law on trafficking and slavery in England and Wales. The Act created two new offences — slavery, servitude and forced or compulsory labour (section 1) and human trafficking (section 2) — and introduced a statutory defence for victims of trafficking who commit criminal offences as a direct consequence of being trafficked (section 45). The Act also established the Independent Anti-Slavery Commissioner and placed the National Referral Mechanism on a statutory footing.
The NRM is the framework through which potential victims of trafficking are identified and referred to support services. A referral is made by a First Responder — which includes the police, local authorities, and certain NGOs — to the Single Competent Authority (SCA), which makes a Reasonable Grounds decision within five working days and a Conclusive Grounds decision within 30 days. A positive Conclusive Grounds decision entitles the victim to a 45-day recovery and reflection period and access to support services.
In criminal proceedings, the section 45 defence provides that a person is not guilty of an offence if they were compelled to commit it, the compulsion was attributable to slavery or relevant exploitation, and a reasonable person in the same situation would have had no realistic alternative. The defence does not apply to a list of excluded offences set out in Schedule 4 to the Act, which includes terrorism and serious sexual offences. The Court of Appeal in R v N; R v Le [2012] EWCA Crim 189 — decided under the predecessor legislation — confirmed that the compulsion defence must be considered by the jury even where the defendant has not formally raised it, if there is evidence to support it.
Expert evidence in modern slavery cases serves three principal functions. First, it provides the decision-maker — whether the SCA, the immigration tribunal, or the criminal court — with the specialist knowledge needed to evaluate the credibility of the victim's account. Second, it explains the psychological mechanisms of trafficking and coercive control that make victims' behaviour appear inconsistent or irrational to those without specialist knowledge. Third, it provides an assessment of the psychological impact of trafficking on the victim and the support they require.
A country of origin expert provides evidence on the prevalence and methods of trafficking in the victim's country of origin, the routes used by traffickers, the typical profiles of victims and perpetrators, and the risk of re-trafficking on return. This evidence is particularly important in NRM cases and in asylum claims where the victim fears return to their country of origin.
A clinical or forensic psychologist assesses the victim's psychological presentation, diagnoses any mental health conditions — including PTSD, complex trauma, and dissociative disorders — and provides an opinion on the consistency of the psychological presentation with the account of trafficking. The expert can also explain the psychological mechanisms of coercive control, trauma bonding, and learned helplessness that explain why victims may have complied with their traffickers' demands.
A physician or forensic medical examiner documents the physical evidence of abuse, including injuries consistent with physical violence, evidence of forced labour, and signs of sexual exploitation. Where the victim claims to have been tortured or subjected to serious physical abuse, a medico-legal report prepared in accordance with the Istanbul Protocol provides the most authoritative assessment of the physical evidence.
A specialist social worker with expertise in modern slavery and trafficking assesses the victim's support needs, their risk of re-trafficking, and the appropriateness of the support services being provided. This evidence is particularly relevant in cases involving children, where the local authority has safeguarding responsibilities.
The section 45 defence raises two questions on which expert evidence is particularly valuable. The first is whether the defendant was a victim of trafficking at the time of the offence. The second is whether the compulsion was attributable to the trafficking — that is, whether the defendant's will was overborne by the trafficker's control to the extent that they had no realistic alternative to committing the offence.
A psychologist who can explain the mechanisms of coercive control — including trauma bonding, the cycle of abuse, and the psychological consequences of prolonged exploitation — provides the jury with the framework needed to evaluate the defendant's account. Without this evidence, juries may apply a standard of rational agency that is inconsistent with the psychological reality of trafficking.
The Court of Appeal in R v MK [2018] EWCA Crim 667 confirmed that a positive NRM decision is relevant but not determinative of the section 45 defence. The court held that the jury must consider all the evidence, including expert evidence, in determining whether the defendant was a victim of trafficking and whether the compulsion defence is made out.
Whether the individual's account is consistent with the typical profile and experience of trafficking victims from their country of origin.
The psychological mechanisms of coercive control and how they apply to the specific facts of the case.
The consistency of the psychological presentation with the account of trafficking.
The impact of the trafficking on the individual's mental health and their ability to exercise free will.
The risk of re-trafficking on return to the country of origin.
The support needs of the individual and the likely impact of different outcomes on their recovery.
The timing of instructions is critical in modern slavery cases. In NRM cases, expert evidence should be obtained before the Conclusive Grounds decision where possible, so that it can be submitted to the SCA as part of the representations. In criminal proceedings, instructions should be given as soon as the section 45 defence is identified — ideally at the pre-charge stage — to allow the expert to assess the defendant before the trial.
The letter of instruction should provide the expert with the individual's full history, the NRM referral and any decisions, the criminal charges (if applicable), the relevant country of origin information, and the specific questions the expert is asked to address. Where a psychological assessment is required, the expert should be provided with any available medical or mental health records.
Re-traumatisation and the assessment process
Victims of trafficking are frequently re-traumatised by the process of recounting their experiences. The expert should be experienced in trauma-informed assessment techniques and should take appropriate steps to minimise distress during the assessment. The letter of instruction should make clear that the expert is expected to conduct the assessment in a trauma-informed manner and to flag any concerns about the individual's welfare to the instructing solicitor.
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