We analyze the incentives properties of the standard of proof for a finding of negligence when courts have imperfect information about injurers' level of care. We show that the 'preponderance of evidence' or 'more likely than not' standard used in common law provides maximal incentives to exercise due care. This extends to situations where unequally informed parties must be led to disclose relevant evidence in an adversarial context and where courts do not know how well informed the parties are. Abstracting from the cost of producing evidence, the optimal assignment of the burden of proof also follows from the 'more likely than not' standard.