A regulative norm for permissible defense distinguishes the conditions under which we will hold defenders to be innocent of any wrongdoing from those in which we hold them responsible for assault or manslaughter. The norm must strike a fair balance between defenders' security, on the one hand, and other agents’ legitimate claim to live without fear of suffering mistaken defensive harm, on the other. Since agents must make defensive decisions under high pressure and on only partial information, they will sometimes make mistakes. We have reason to want a norm that considers a mistake permissible when it was highly likely on the evidence that defense was proportionate and necessary to avert a threat. However, adopting an evidentialist norm under non-ideal conditions is treacherous business. I briefly survey empirical data suggesting that the type and extent of bias prevalent in the US renders a straightforward evidentialist norm unjust, and thus since the legal practice in the US relies on such a norm, we must explore avenues for reform. Preferably this will take the form of adopting a modified evidential norm, and I explore some promising options. If this proves impossible, however, then we have to accept a strict regulative norm (which does not consider any mistakes permissible), as the sole just alternative.