The “Unknown Male DNA” Rule: Post‑conviction DNA identifying a non‑defendant donor in a sexual homicide renders a historic conviction unsafe

Introduction

This commentary examines the Court of Appeal’s decision in Sullivan, R v [2025] EWCA Crim 772, a landmark in post‑conviction review jurisprudence where modern DNA techniques conclusively excluded the appellant as the source of semen recovered from the victim in a sexual homicide. The case revisits a 1987 murder conviction arising from the brutal killing of 21‑year‑old Diane Sindall in Birkenhead.

The original prosecution was circumstantial. Key strands included: alleged bite marks attributed to the appellant; a borrowed crowbar that could have caused the injuries; fibre evidence of limited probative value; assertions of incriminating admissions during police interviews (some made before continuous legal access and recording were guaranteed in practice); and post‑offence conduct said to implicate the appellant (presence near Bidston Hill, where the victim’s belongings were later burned).

Decades later, the Criminal Cases Review Commission (CCRC) referred the conviction to the Court of Appeal following powerful fresh DNA analysis demonstrating that semen on the victim’s body was from an unknown male—neither the appellant, Peter Sullivan, nor the victim’s fiancé. Additional fresh evidence was also proffered: expert critique of bite mark identification and psychological evidence challenging the reliability of confessions. The court admitted the DNA evidence under section 23 of the Criminal Appeal Act 1968 and held that, in light of that evidence, the conviction was unsafe. It declined to admit the bite mark and confession evidence as unnecessary to the outcome. The Crown did not seek a retrial; the conviction was quashed and Sullivan was ordered to be released.

Summary of the Judgment

• The court accepted fresh DNA evidence showing semen on Ms Sindall’s body originated from an unknown male (not the appellant or fiancé). It was common ground that the attack had a sexual aspect and there was no evidence of multiple offenders or consensual sexual activity proximate to the homicide.
• Applying section 23 of the Criminal Appeal Act 1968, the court held the DNA evidence was capable of belief, admissible, provided a reasonable explanation for its late emergence (due to scientific advances), and afforded a ground for allowing the appeal. Crucially, if this DNA evidence had been available in 1986, the overall evidence would have been insufficient even to charge the appellant (para 42).
• The court declined to receive as fresh evidence the new bite mark and confession-reliability material. Standing alone, each would not have afforded a ground of appeal; and in any event, once the DNA evidence was admitted, it governed the outcome.
• The conviction was quashed. The respondent did not seek a retrial; the appellant was ordered to be released.

Detailed Analysis

1) Precedents and Framework Cited

• Criminal Appeal Act 1968, section 23: The statutory gateway for receiving fresh evidence on appeal. The court recited and applied the four key considerations:
  • Capability of belief;
  • Whether the evidence may afford any ground for allowing the appeal;
  • Admissibility at trial on a live issue;
  • Reasonable explanation for failure to adduce earlier.
• Police and Criminal Evidence Act 1984 (PACE) and Code of Practice: Referenced in assessing historical interview practices (including permissible temporary denial of legal advice at the time) and the defence’s ability at trial to challenge compliance.
• The 2021 Court of Appeal decision in the appellant’s first appeal attempt: The court recalled its prior conclusions (in 2021) that, in isolation, bite mark critique and reliability-of-confession evidence did not render the conviction unsafe. Those conclusions still stood in 2025 regarding those issues taken alone.
• CCRC role: The referral mechanism was central; the CCRC’s 2008 decision not to refer (given then‑available science) was deemed plainly correct, whereas scientific advances by 2025 justified referral and admission.

2) The Court’s Legal Reasoning

The court’s analysis proceeded in two stages: admission of fresh evidence under s23, then the safety of the conviction in light of that evidence.

• Admission under section 23: The court found the DNA statements by the forensic scientists “plainly capable of belief” and admissible if available at trial. The only reason they were unavailable was the absence of scientific techniques in 1986–87; that constituted a “reasonable explanation” for failure to adduce earlier (para 42). Because the fresh DNA could “afford a ground for allowing the appeal”—indeed, decisively so—admission was “necessary and expedient in the interests of justice” (para 43).
• Probative force of unknown-male semen in a sexual homicide: The court underscored the strength of the inference that the attacker ejaculated on the victim. With no evidence of multiple offenders and no consensual explanation, DNA identifying a non‑defendant as semen donor powerfully points to an “unknown man” as the attacker (para 42). This inference was so compelling that—had it been available in 1986—the case would have been insufficient even for charge.
• Global reassessment of circumstantial evidence: The court emphasized that the new DNA evidence “inevitably weakens each strand of the circumstantial evidence presented at trial” (para 49). That is, post‑conviction DNA can reframe the entire evidential mosaic, reducing the probative value of other strands (bite marks, fibres, alleged confessions, presence near the burn site) even if each had appeared persuasive at the time.
• Economy in deciding ancillary grounds: Having found the DNA grounds determinative, the court took grounds 2 (bite mark) and 3 (confessions) “quite shortly” (para 44), declining to admit them as fresh evidence and reaffirming that, in isolation, neither would have justified allowing the appeal (paras 44–48).

3) Bite Mark Evidence: From “ID” to (at most) “circumstantial feature”

• At trial, oral pathology and forensic odontology evidence—hotly contested—supported attribution of bite marks to the appellant. That evidence carried weight in the original circumstantial case.
• By 2025, the British Association of Forensic Odontology (and the US analogue) no longer endorsed positive identification by bite marks. Professor Pretty’s evidence explained the absence of a scientific basis for identification.
• The court nevertheless distinguished between admissibility and weight. It noted that bite marks, while no longer suitable for positive identification, might still be used as one circumstance within a broader circumstantial narrative. The court did not announce a categorical exclusionary rule. Instead, it held that Professor Pretty’s new material, standing alone, would not have afforded a ground for appeal, particularly given that the bite mark evidence was extensively challenged at trial and reconsidered in the 2021 appellate decision (paras 45–46).

4) Confession Reliability: Historic Context and Limited Incremental Value

• The appellant gave inconsistent accounts, with episodes of apparent admission and retraction. He alleged misconduct and pressure; he was at times denied immediate legal advice, which was permissible under the then‑operative PACE Code.
• At trial, admissibility was ruled upon, and defence counsel cross‑examined broadly on reliability, including cognitive vulnerabilities. In 2021, the Court of Appeal had already considered expert psychological opinion about suggestibility and compliance.
• By 2025, Professor Shepherd’s report—critiquing interview practices and emphasizing vulnerability—was treated as largely duplicative or of doubtful admissibility insofar as it ventured into evaluating police conduct. The court held that, independently, it would not have afforded a ground to allow the appeal (paras 47–48).

5) A Notable Metric: The “Charging Counterfactual”

A striking feature of the reasoning is the court’s express statement that, had the fresh DNA been available in 1986, the evidence as a whole would have been insufficient even to charge the appellant (para 42). This “charging counterfactual” serves as a particularly emphatic way to articulate why the conviction is unsafe. While not a freestanding legal test, it is a strong indicator of how courts may frame the probative weight of post‑conviction DNA that points to a specific alternative perpetrator (an “unknown man”) in sexual homicide scenarios without competing explanations.

Impact and Significance

1) The “Unknown Male DNA” Rule in Sexual Homicides

• Core principle: Where a homicide exhibits a sexual aspect and fresh DNA identifies semen from an unknown male (exclude the defendant and consensual partner), the conviction will ordinarily be unsafe, absent evidence of multiple offenders or credible consensual explanations. The DNA’s exculpatory force is sufficient to overwhelm a circumstantial case that might otherwise have seemed strong.
• Scope: This is most potent in single‑perpetrator sexual attacks and homicides. The lack of evidence of multiple assailants is key. If the record contained credible evidence of multiple perpetrators, the calculus might differ.

2) Practical Consequences for CCRC Reviews and Historic Cases

• Renewed testing is justified: Cases where biological material was recovered but not viable for profiling at the time should be reassessed given incremental advances in DNA technology. The court endorsed the CCRC’s prudence both in 2008 (when testing would likely have destroyed samples without probative yield) and in 2025 (when testing was probative).
• Reasonable explanation for late evidence: Scientific progress readily satisfies section 23(2)(d). Applicants hindered by prior technology can meet the “fresh evidence” gateway without being penalized for delay.

3) Bite Mark Evidence Going Forward

• No per se inadmissibility: The court did not declare bite marks inadmissible. Rather, it recognized professional bodies’ non‑endorsement of positive identification and treated bite marks—if relied upon at all—as a low‑weight circumstantial feature subject to robust challenge.
• Prosecution practice: Prosecutors should be cautious in treating bite mark analysis as identification evidence. At most, it might form part of a circumstantial matrix, and only with clear directions about its limitations.

4) Confessions and Interview Practices

• Historic interviews contextualized: The judgment situates confession evidence within the then‑legal environment under PACE. The court remains open to psychological vulnerability evidence but emphasizes proper boundaries on expert commentary about police conduct and the need to focus on admissibility and weight within applicable law at the time.
• DNA eclipsing other evidence: Even if confessional inconsistencies were admissible and probative, exculpatory DNA of the strength seen here will likely dominate the safety assessment.

5) Appellate Case Management: Admitting Only What Is Necessary

• Where one strand of fresh evidence is determinative, the court may decline to admit other new material that, standing alone, would not justify allowing the appeal (paras 44–50). This avoids advisory pronouncements on evolving scientific controversies (such as bite mark identification) when the case can be resolved on firmer ground.
• Respondents may properly refrain from seeking retrials where the post‑conviction DNA leaves the case non‑viable. Here, the Crown did not oppose the appeal and did not seek a retrial.

Complex Concepts Simplified

• “Safe” conviction: In England and Wales, an appeal succeeds if a conviction is “unsafe.” That means the court has real doubt about its reliability—because of legal error, unfairness, or new evidence that undermines confidence in the verdict.
• Section 23 Criminal Appeal Act 1968 (fresh evidence test): The court may receive new evidence if:
  • it is believable;
  • it could provide a reason to allow the appeal;
  • it would have been admissible at trial on the relevant issue; and
  • there is a reasonable explanation for not presenting it at trial (e.g., technology did not exist).
• CCRC referral: The Criminal Cases Review Commission is an independent body that investigates potential miscarriages of justice and can refer convictions to the Court of Appeal when new evidence or arguments provide a real possibility the conviction will be quashed.
• Circumstantial evidence: Evidence that implies a fact but does not directly prove it (e.g., presence near a scene, possession of potential weapons, fibres). Individually weak strands can collectively appear strong—until strong direct scientific evidence (e.g., exculpatory DNA) changes the picture.
• Bite mark evidence: Forensic comparison of injuries to dentition. Professional consensus has shifted: positive identification is no longer endorsed by leading bodies. Courts may still admit bite mark observations as context, but with caution about reliability.
• Confession reliability: Courts evaluate whether statements were obtained and given fairly and whether they are reliable, considering legal safeguards (like access to legal advice) and personal factors (like suggestibility and cognitive limitations).

Key Takeaways

• Decisive rule for sexual homicides: Fresh DNA identifying semen from an unknown male in a sexual homicide, absent evidence of multiple assailants or consensual alternatives, will ordinarily render a conviction unsafe. This is the clearest doctrinal signal in the judgment.
• Section 23’s flexibility validated: The court reaffirmed that scientific advances provide a “reasonable explanation” for late evidence and that admission is “necessary and expedient” when such evidence fundamentally alters the case.
• Bite mark identification downgraded: While not excluded per se, bite mark analysis is no longer generally suitable for positive identification and should be treated, at most, as a modest circumstantial feature.
• Confession evidence assessed in its historical legal context: The court resists converting modern standards into retroactive grounds of exclusion where the trial judge ruled on admissibility and the defence probed reliability.
• Global reassessment principle: Powerful fresh DNA evidence compels a holistic re‑evaluation of all other evidence; it can substantially erode the probative value of circumstantial strands that once appeared compelling.
• Procedural economy: Where one ground is dispositive, the court may decline to admit or determine other fresh evidence grounds.

Conclusion

Sullivan, R v crystallizes a potent and pragmatic principle for post‑conviction review in sexual homicides: unknown‑male semen DNA—where there is a strong sexual aspect and no credible multiple‑offender or consensual narrative—undermines the safety of a conviction, even where the original case was built from numerous circumstantial strands. The decision illustrates the measured operation of section 23 of the Criminal Appeal Act 1968, validating the admission of transformative scientific evidence while resisting unnecessary forays into contested forensic and psychological debates where they do not affect the result.

The judgment also refines practice: prosecutors should treat bite mark analysis with circumspection; defence teams and the CCRC should prioritize re‑testing where advances may now yield probative results; and appellate courts will undertake a global reassessment of the evidential picture in light of decisive scientific findings. Above all, the case affirms the justice system’s capacity to correct historic convictions when new science reveals that, as here, an “unknown man” was almost certainly the perpetrator.