DEFENCE THEORY & FORENSIC ANALYSIS

Alternative Perpetrator and Forensic Gaps

An alternative-perpetrator theory is a defence proposition that another person, rather than the defendant, may have committed the alleged act.

English criminal law does not generally require the defence to identify or prove a specific alternative perpetrator. The central question remains whether the prosecution has established the defendant’s guilt to the required criminal standard on the evidence as a whole.

THE FORENSIC GAP

Opportunity Is Not Identification

In a circumstantial case, physical presence, opportunity, proximity, motive and other surrounding circumstances may form part of the prosecution evidence.

But opportunity to commit an act is not the same proposition as proof that the defendant committed it. The evidential chain must still connect the defendant to the alleged conduct.

The absence of a proven alternative perpetrator does not automatically establish the defendant as the perpetrator.

The relevant question is whether the evidence identifying the defendant is sufficiently strong when the competing circumstances and evidential gaps are considered together.

CIRCUMSTANTIAL EVIDENCE

Gaps Must Not Be Filled by Speculation

Circumstantial evidence may be powerful when separate circumstances combine to support a conclusion of guilt. The fact that evidence is circumstantial does not make it inherently weaker than direct evidence.

The difficulty arises when an evidential gap is resolved not by proved circumstances but by an assumption about what must have happened.

Judicial College guidance states that juries must examine the strands of circumstantial evidence, decide which they accept, and determine what fair and reasonable conclusions can be drawn from them. They must not speculate or guess about matters that have not been proved by evidence.

THE ALTERNATIVE-PERSON HYPOTHESIS

What Must Be Tested?

The defence does not necessarily need to prove that Party B committed the offence. It may be sufficient to identify evidence that materially weakens the inference that Party A must have committed it.

For example, where another person had physical access to the relevant location, was present during the critical period, or was involved in a documented conflict with the alleged victim, those circumstances may be relevant to the competing explanations of the event.

Their significance depends on the evidence. A possible alternative perpetrator is not automatically a reasonable alternative explanation. Equally, the failure to identify that person’s involvement does not automatically eliminate the possibility.

FORENSIC RECONSTRUCTION

When Physical Access Matters

In an allegation involving a particular object, location or physical mechanism, the alternative-perpetrator analysis should be connected to the physical evidence.

The relevant questions may include who could physically access the object, who could occupy the relevant position, what movements were possible, whether the alleged mechanism is compatible with the documented scene, and whether the available evidence excludes or supports competing physical sequences.

A theory that one person was the only possible perpetrator should therefore be treated as a conclusion requiring evidential support, not as a premise from which guilt can simply be inferred.

ANALYTICAL CHECK

Testing the Alternative-Perpetrator Question

What evidence directly connects the defendant to the alleged act?
What evidence establishes the defendant’s physical opportunity?
Who else had physical access to the relevant location or object?
Were other persons present during the critical period?
Did other persons have relevant relationships, conflicts or incentives?
What evidence excludes or weakens competing physical sequences?
Are alleged physical constraints supported by photographs, measurements or expert evidence where required?
Which parts of the prosecution account are established facts and which are inferred?
Has an evidential gap been filled by evidence or by speculation?
Does the evidence as a whole make the tribunal sure of the defendant’s guilt?

IMPORTANT DISTINCTION

“No One Else Could Have Done It” Is a Conclusion, Not a Starting Point

A court may reject an alternative-perpetrator theory after considering all the evidence. But the proposition that no one else could have committed the act must itself be supported by the evidence.

The criminal standard is not whether the defendant appears to be the most likely person responsible. The question is whether, having considered the evidence as a whole, the prosecution has proved guilt so that the tribunal is sure.

This distinction becomes particularly important where the prosecution case depends substantially on circumstantial evidence and the defence identifies unresolved physical, chronological or evidential gaps.

REFERENCES & PRIMARY MATERIALS

JUDICIAL GUIDANCE	Judicial College, Crown Court Compendium, Part I, Chapter 10 — Circumstantial Evidence, March 2026 update
LEADING AUTHORITY	Teper v The Queen [1952] UKPC 15 — circumstantial evidence and the need to examine co-existing circumstances that may weaken an inference of guilt
APPELLATE AUTHORITY	R v Kelly [2015] EWCA Crim 817 — circumstantial evidence, inference and the danger of speculation
SCOPE	England and Wales criminal proceedings; circumstantial evidence, competing hypotheses and evidential gaps.

Source note: An alternative-perpetrator theory is an evidential hypothesis, not a requirement that the defence identify the actual offender. Its significance depends on the evidence and on whether the prosecution has nevertheless proved the defendant’s guilt to the required standard.

The question is not whether another perpetrator has been proved. The question is whether the prosecution has proved this defendant.