"Beyond reasonable doubt" is a standard of evidence, it doesn't have anything to do with whether the jury needs to be unanimous. They need to be unanimous regardless of the standard of evidence, even for "preponderance of the evidence" (which means that the jury finds it more likely than not -- that the evidence "tips the scale"). If the jury is not unanimous they need to deliberate more until they are unanimous (if they deadlock then a new jury is called and the whole case starts again).

The two concepts seem related, at least if we assume that jurors are reasonable. A single reasonable juror having a doubt indicates that the verdict is not beyond reasonable doubt. I agree that unanimity is not as obvious when the standard is "preponderance".

I guess I should've said that it's a separate condition, rather than being unrelated. The standard each juror needs to consider when making their own determination is "beyond reasonable doubt" (or "preponderance" or "clear and convincing"), and in order for the verdict to be held by the jury it needs to be unanimous.

I think the assumption that jurors are invariably reasonable, as opposed to on average reasonable is a pretty heroic assumption not well supported by the history of trial by jury though.

A jury selection process lets both sides of a case interview jurors and agree on impartial and reasonable jurors.

Estimates of wrongful conviction are between .13%-5% (wikipedia, miscarriage of justice), however the main thing that trial by jury affords is transparency and perception of fairness, since you are convicted by your peers and not some politician with questionable incentives.

Don't forget that the court decides what a jury gets to see or hear about a case. Juries are not perfect, but they are the best tool we have.

Further, the prosecutor has real power that far outweighs the power of the accused. Juries imperfectly provide an offset to that power imbalance.

And yet, the US legal system is built around such assumptions, invalid as they may be.

At the risk of being a bit technical, (as law tends to be) I think reasonable doubt is called the "standard of evidence". Its the phrase "jury trial" that the court found to require unanimous verdicts.

Criminal jury trials have the unanimity requirement. Also, criminal convictions are held the "beyond a reasonable doubt" standard, though there may be collateral actions, or procedural determinations in a criminal trial that have a different standard of evidence.

Otherwise, US States have a lot of flexibility in how they handle non-criminal cases.