Normative legal argument based on empirical evidence is not necessarily best served by standards from the social sciences. Precautionary concern for false negatives may call for an adjustment of the significance level. That all legal choice is historically contingent, that legal problems tend to be ill-defined, and that strategic actors have an incentive to bias the generation of evidence create further challenges. Yet the law can capitalize on the adversarial principle. Competition among interested parties helps contain the strategic element and spurs the creative search for better evidence. This leads to suggestive, but institutionally contained, empirical evidence.