Why do we require evidence beyond a reasonable doubt?

Straight out of David Friedman's book, Laws Order.

In fact, law in the United States and similar systems require a high standard of proof ("beyond a reasonable doubt") in a criminal case but only a low standard ("preponderance of the evidence") in a civil case. Why? The answer cannot simply be that we are more careful with criminal convictions because the penalties are bigger. A damage judgment of a million dollars, after all, is a considerably more severe punishment for most of us than a week in jail.

Economics suggests a simple explanation. The typical result of losing a lawsuit is a cash payment from the defendant to the plaintiff. The result of being convicted of a crime may well be imprisonment or execution. A high error rate in civil cases means that sometimes I lose a case I should have won and pay you some money and sometimes you lose a case you should have won and pay me some money. On average the punishment itself imposes no net cost; it is simply a transfer. A high error rate in criminal cases means that sometimes I get hanged for murder I didn't commit and sometimes you get hanged for a murder you didn't commit. In a criminal case, unlike a civil case, one man's loss is not another man's gain. Punishment is mostly a net cost rather than a transfer, so it makes sense to be a good deal more careful about imposing it.

I don't think the makers of our institutions had this in mind, so it is strange that we got such a conducive outcome by accident. The same could be said of a lot instances found in David Friedman's book. There are far more efficiencies than we would first expect when we consider how little of our system was planned and how much of it evolved. We betray this lack of intent when we say, "the reason why" when we mean, "the way things turned out."

Moreover, when it was planned, much of it was done with completely different rationales from the justifications we would give today. I notice this in much of modern economic theory, where economists have re-justified policies with expired rationales.

Also interesting is this paper which asks which standard of proof should be used when criminal conduct has to be proved in civil proceedings. It seems to me like a court trying a civil case should be able to assume a "found innocent" person is guilty if the evidence suggests as much.

But what do I know.