What does it mean for a defendent to be found guilty “beyond reasonable doubt“? The courts aren’t telling. Not in the UK, not in the US, not in any of the other countries where it’s the basic standard of criminal justice. The US Supreme Court thinks that “Attempts to explain the term “reasonable doubt” do not usually result in making it any clearer to the minds of the jury“. So the most you get is state-level definitions, such as that of California:
It is not a mere possible doubt; because everything relating to human affairs, and depending on moral evidence, is open to possible doubt. It is that state of the case which, after the entire comparison and consideration of all the evidence, leaves the minds of the jurors in that condition that they cannot say they feel an abiding conviction, to a moral certainty, of the truth of the charge.
The courts, in particular, are vehemently opposed to any attempt to put a numerical probability to beyond reasonable doubt. It requires more than a 50% likelihood of guilt, and less than 100%. But between those, it’s anybody’s guess.
So what happens when you ask people? Teach them some basic probability, give them a definition of “beyond reasonable doubt”? It’s been tried. A fifth of people interpret it as 90%, with slightly lower numbers choosing 95% or 99%. But fully 27% think 80% certainty, or less, would be enough to convict.
Another experiment asked half of a group to act as jurors, declaring defendents guilty or not. The other half put a numerical figure on how likely they were to be guilty. Matching up the decisions of the two groups should show what probability corresponds to “beyond reasonable doubt”. Here it seems a guilty verdict requires 0.70 – 0.74 probability of guilt.