When can new evidence after conviction be heard on appeal?

A jury's verdict is meant to settle a criminal case. Sometimes, though, material comes to light after the trial that the jury never saw: a new witness, a retraction, a scientific advance or a document that was not disclosed. Whether an appeal court will look at it depends on strict legal tests that balance two concerns: the finality of verdicts and the risk of leaving a wrongful conviction in place.

Why appeal courts are cautious

An appeal is not a second trial. Appeal judges normally review the case on the evidence the trial court heard, and they ask whether legal errors were made. They do not usually weigh the evidence afresh.

There is also a practical reason for caution. If defendants could hold back evidence and then produce it only after a guilty verdict, trials would lose much of their purpose. Courts therefore want to know why the material was not put forward earlier, and whether it is reliable enough to matter.

England and Wales: the interests of justice

In England and Wales, the Court of Appeal has a statutory power under the Criminal Appeal Act 1968 to receive evidence when it considers this necessary or expedient in the interests of justice. The Act directs the court to have regard in particular to several questions:

• Does the evidence appear capable of belief?
• Might it afford a ground for allowing the appeal?
• Would it have been admissible at the trial?
• Is there a reasonable explanation for the failure to produce it then?

These factors guide the court rather than operate as rigid conditions. The judges have discretion, and an explanation for the delay that is weak does not always prevent the evidence being heard if it raises real doubt about guilt.

The central question is whether the conviction is "unsafe". The court asks whether the new material, taken with the rest of the case, casts enough doubt on the verdict. The judges may also hear from the witness in person before deciding.

The role of the review commission

Many cases in which new evidence after conviction is relied on reach the Court of Appeal through the Criminal Cases Review Commission, an independent body that investigates alleged wrongful convictions in England and Wales, Northern Ireland and, in some circumstances, Scotland through its own commission. It can refer a case when it considers there is a real possibility that the conviction would not be upheld, which generally requires new argument or evidence. A referral does not guarantee success, but it brings the case before the court.

The United States: newly discovered evidence

American practice varies by state, and federal courts have their own rules. Under the federal rules of criminal procedure, a defendant can ask the trial court for a new trial on the basis of newly discovered evidence, normally within three years of the verdict. Courts commonly expect the defendant to show that:

1. the evidence was discovered after the trial;
2. the defendant was diligent in trying to find it earlier;
3. it is not merely cumulative or aimed only at undermining a witness;
4. it is material; and
5. it would probably lead to an acquittal at a retrial.

State systems have their own deadlines and standards, and many offer separate post-conviction procedures. Some states have passed laws allowing convicted people to ask for DNA testing of biological material or to challenge convictions that relied on forensic methods since discredited. Claims of actual innocence that rest on fresh evidence are especially hard to bring in federal habeas proceedings, where procedural barriers can be severe. A convincing showing of innocence can sometimes open a route past those barriers.

Canada: the Palmer approach

Canadian appeal courts have long applied principles drawn from the Supreme Court's decision in R v Palmer. The court asks whether the evidence could have been obtained with reasonable diligence at trial, whether it bears on a decisive issue, whether it is credible, and whether, taken with the trial evidence, it could reasonably be expected to have affected the result. The first requirement can be relaxed in criminal cases where the interests of justice demand it.

Separately, the Criminal Code allows the federal Minister of Justice to review claims of wrongful conviction and, where warranted, order a new trial or send the matter to an appeal court.

Scientific evidence and DNA

DNA testing has reshaped this area of law. A DNA exoneration typically happens when testing of preserved material, using methods not available at the original trial, excludes the convicted person as the source of biological evidence. Courts generally treat such results as strong because the science is well established, but they still ask how the result fits with the remaining evidence. A DNA profile that does not match may not by itself undermine a case that rested on other proof.

Scientific developments can also work the other way, raising doubts about forensic techniques that were once accepted. Courts then have to decide whether a change in scientific opinion amounts to fresh evidence or only a new view of old evidence.

What happens if the evidence is admitted

Admitting fresh evidence is not the same as quashing a conviction. The appeal court may dismiss the appeal, quash the conviction, or, in some systems, order a retrial. Where a conviction is overturned, the prosecution may be able to seek a new trial if the interests of justice require it, though rules on double jeopardy differ between countries.

Why the threshold matters

Each legal system tries to avoid two failures: reopening cases on weak or unreliable claims, and shutting out evidence that shows a miscarriage of justice. The tests on diligence, credibility and impact on the verdict are the tools courts use to draw that line. They differ in wording, but in each country the appeal court is asking the same underlying question: whether the verdict can still be trusted in light of what is now known.