Why Understanding Forensic Biology Evidence Matters in Sexual Offence Cases

The updated CPS RASSO guidance places renewed emphasis on robust case strategy and the effective use of expert evidence. We consider what this means when forensic biology forms part of a case. 

The Crown Prosecution Service’s recently updated Rape and Serious Sexual Offences (RASSO) Guidance reflects a continued commitment to strengthening the investigation and prosecution of rape and serious sexual offence cases.  

The revised guidance places renewed emphasis on building cases that are evidentially strong, suspect-centred and victim focused. Key areas include: 

• Developing a robust case strategy and identifying the most appropriate charges.  

• Challenging misconceptions and assumptions about rape and sexual offences. 

• Considering suspect behaviour, patterns of misconduct and other relevant evidence. 

• Managing reasonable lines of enquiry, disclosure and the legal issues arising in RASSO cases. 

• Making effective use of expert evidence where specialist interpretation is required. 

The update comes alongside wider developments in the CPS’s approach to RASSO cases, including the introduction of the Early Victims’ Right to Review (VRR) pilot. The pilot gives eligible individuals the opportunity to request a review by a second prosecutor before a prosecution is formally stopped in court. 

A Wider Focus on Building Stronger Cases 

Together, these developments reflect a broader commitment to improving the quality of decision-making in rape and serious sexual offence cases and ensuring that the available evidence is assessed thoroughly and appropriately. 

Central to the updated guidance is the importance of developing a robust case strategy. This includes considering the evidence as a whole, identifying the most appropriate charges and ensuring that the case presented reflects the available evidence. 

Alongside this is the need to challenge misconceptions about rape and sexual offences. Assumptions about how a victim might behave before, during or following an alleged offence can influence how evidence is perceived. The guidance encourages prosecutors to identify and address such misconceptions and to consider the circumstances of each case on their own terms. 

Scientific evidence can form an important part of this wider evidential picture. In sexual offence cases, this frequently includes forensic biology, from body fluid examination and DNA profiling to the interpretation of complex scientific findings. Ensuring that this evidence is properly understood and assessed also plays an important role in maintaining confidence in the criminal justice system. 

The Role of Forensic Biology in Sexual Offence Investigations 

Forensic biology extends far beyond simply identifying the presence of DNA. It encompasses the examination of body fluids, the interpretation of DNA findings and the scientific evaluation of how those findings relate to the circumstances of a case. 

This may include the examination of clothing, intimate swabs or other exhibits recovered during an investigation. DNA profiles may help identify the source of biological material, while specialist techniques such as Y-STR analysis can assist in cases involving smaller quantities of male DNA in samples dominated by that of the female. Scientists may also consider whether the findings provide support for one account of events over another, always recognising the limitations of the available evidence. 

Equally, the absence of semen does not necessarily mean that sexual contact did not take place. Factors such as the use of a condom, ejaculation outside the body, washing, urination and the time between the alleged incident and medical examination can all affect whether semen is detected. A negative result must therefore be considered carefully and within the full context of the allegation. 

Understanding the Evidential Value of Forensic Biology 

A common misconception surrounding forensic biology evidence is that the detection of DNA automatically answers the key questions in a case. In reality, forensic science is rarely that straightforward. 

A DNA profile may indicate whose biological material is present, but it cannot necessarily determine when that material was deposited or precisely how it came to be there. Detecting a person’s DNA on clothing or another item does not automatically establish direct contact, as transfer, previous contact, washing and environmental conditions may all influence the findings.  

The location, quantity and type of DNA recovered must therefore be interpreted alongside the circumstances of the case. Mixed DNA profiles, body fluid identification and other complex scientific findings require careful evaluation by experienced forensic scientists. 

The management of reasonable lines of enquiry sits alongside the wider disclosure and legal considerations addressed within the updated guidance. In this context, a clear understanding of the scientific evidence is essential. Legal professionals need to recognise what forensic findings can and cannot demonstrate and ensure that any conclusions drawn are supported by the forensic material available in the case. 

The role of the forensic scientist is not to determine guilt or innocence, but to provide an objective evaluation of the scientific findings and explain their evidential significance. This includes setting out the extent to which the findings may support one proposition over another, while clearly identifying any limitations and ensuring that conclusions remain proportionate to the available evidence. 

Understanding these principles helps legal professionals ask informed questions, engage effectively with expert reports and ensure that forensic biology evidence is presented or challenged appropriately. 

The Importance of Expert Interpretation 

As forensic techniques continue to evolve, expert interpretation becomes increasingly important. Advances in DNA analysis have increased the sensitivity of forensic testing, allowing scientists to recover and analyse increasingly small quantities of biological material.  

While this creates greater opportunities to obtain potentially useful evidence, increased sensitivity does not necessarily make interpretation any easier. It can make questions around how DNA may have been transferred, how long it may persist and the significance of a finding particularly important. 

Mixed DNA profiles can present challenges. A sample may contain DNA from more than one person, sometimes at very different levels, which can make the profile more complex to evaluate. The scientist must assess the components of the mixture, the strength of the findings and their significance in relation to the questions raised by the case. 

Scientific evidence is therefore not simply about generating results. The expert must explain the significance of those results, consider alternative explanations and communicate complex findings clearly and objectively to investigators, solicitors, barristers and the court. 

Whether instructed by the prosecution or the defence, the forensic scientist’s duty remains the same: to provide an independent, objective opinion based on the scientific findings and the available evidence. 

This is particularly important in rape and serious sexual offence cases, where forensic biology evidence often forms one part of a much broader, complex evidential picture. 

The updated RASSO guidance reinforces the importance of making effective use of expert evidence where specialist interpretation is required. In forensic biology cases, this means ensuring that complex scientific findings are explained clearly to the court and that expert evidence is used appropriately within the prosecution process. 

Specialist Training and Independent Forensic Expertise 

At Forensic Access, our scientists bring extensive practical case experience to both expert witness work and specialist training, including our Forensic Biology in Sexual Assault Cases training for legal professionals.  

The training provides practical guidance on interpreting forensic biology evidence, understanding forensic scientist reports and considering scientific findings throughout the prosecution and defence process. Topics include body fluid examination, DNA profiling, mixed DNA profiles, Y-STR analysis and the interpretation of forensic evidence in sexual offence investigations. 

Forensic Access provides access to in-house forensic biology experts who can review existing forensic findings and scientist reports, assess the interpretation of DNA and other biological evidence, identify areas that may require further scientific consideration, conduct re-examinations and new examinations at our laboratories and act as independent expert witnesses for court. 

If you require an independent forensic biology expert, would like to discuss a case or are interested in specialist training for your team, please contact our Casework Team on 01235 774870 or email science@forensic-access.co.uk.