We study and compare law enforcement costs under two alternative legal presumptions, one more pro-defendant than the other, with the objective of reducing crime to a target level. We identify relative strengths and weaknesses of these legal presumptions in terms of components of law enforcement costs such as costs, costs, trial costs and (when the potential offender is an official) agent costs. We show that relatively pro-defendant presumptions have several cost advantages, but some of these advantages disappear if law enforcers never collude with potential offenders or if resources to motivate law enforcers are limited. Our approach is fundamentally different form the literature in that we shift the focus from the ex-post trial stage to the ex-ante stage of a criminal decision and analyze the role of legal presumptions in crime prevention. We show that when law enforcers are incorruptible and resources to compensate successful law enforcers are not bounded above, the relatively pro-defendant legal presumption allows implementation of all crime levels at lower expected costs. If resources to compensate successful law enforcers are limited, there is a range of low crime levels that can be implemented only by switching from the pro-defendant to the pro-prosecution legal presumption. Finally, if the principal has an elite force of honest and non-collusive law enforcers, or if collusion between law enforcers and potential criminals is impossible or very costly to sustain, collusion prevention costs are zero. Agent compensation costs are also zero if the potential offender is not an employee of the principal. Then the choice between the two legal presumptions rests on expected trial costs and evidence production costs. We show that evidence production costs in this case are determined by a simple balance between the likelihood of type-I and type-II errors. If shifting the burden of proof to criminal defendants increases the probability of avoiding type-II error by more than the probability of committing type-I error, accuracy in adjudication increases, and a stronger deterrence, hence lower expected evidence production costs, will obtain for any target level of crime.