What is the difference between 'beyond reasonable doubt' and 'the balance of probabilities'?

They are the two standards of proof in English law, and the gap between them explains how the same events can produce an acquittal in a criminal court and a finding of liability in a civil one.

Beyond reasonable doubt — the criminal standard. The prosecution must make the jury or magistrates sure of guilt. Modern judicial directions increasingly use the word "sure" rather than the traditional phrase, because juries found the older formulation harder to apply.

It does not mean beyond all doubt or beyond any imaginable possibility. It means no doubt that a reasonable person would act upon.

Balance of probabilities — the civil standard. The claimant must show their case is more likely than not — conventionally described as anything above 50%. If the court concludes it probably happened, that is sufficient.

Why criminal cases demand more. The consequences — imprisonment, a criminal record, loss of liberty — are of a different order from a financial judgment, and the state is bringing the case with vastly greater resources than the individual. The high standard reflects the principle, expressed by Blackstone, that it is better for guilty people to go free than for an innocent person to be convicted.

Why this produces apparently contradictory outcomes. A defendant acquitted of a criminal offence can still be found liable in civil proceedings on the same facts — the jury was not sure, but a civil court finds it more probable than not. This is not the system malfunctioning; the two courts answered different questions.

The burden of proof — who must prove it. In criminal cases the burden lies on the prosecution throughout. A defendant need not prove innocence, and is entitled to say nothing. This is the presumption of innocence, and the phrase "the defence failed to explain" misunderstands it. Certain limited reverse burdens exist, where a statutory defence must be established by the defendant, on the balance of probabilities.

In civil cases the burden generally lies on whoever asserts something.

Other contexts. Tribunals, regulatory bodies and professional disciplinary panels generally apply the civil standard, which is why a professional can be struck off on facts that produced no prosecution.

General information, not legal advice.