Adverse inference is a legal situation in which someone making a judgment concludes that evidence was not produced because it would be unfavorable to the party being asked to produce evidence. In this particular case, silence or refusal to cooperate is taken as a suggestion that wrongdoing occurred and someone tried to cover it up. Adverse inference is most often seen in civil cases.
A judge can opt to issue what is known as an adverse inference instruction to the jury. In this case, the judge informs the jury that someone did not produce evidence, or spoiled the evidence so that it could not be brought to court, with the intention of concealing information which might hurt his or her case. This instruction is only undertaken when there is strong evidence to support adverse inference; if it cannot be proved that the evidence was relevant, the judge cannot alert the jury to the situation.
An adverse inference sanction is very serious. Juries can only be given such instructions when it is clear that spoilage or concealment of evidence occurred, and that the goal was to cover up unfavorable materials. If it can be demonstrated that the event occurred by accident or that the evidence was not relevant to the case, no adverse inference can be made. These measures are designed to protect people by ensuring that they cannot be incriminated by innocent actions. In the example above, for example, Mary might have reformatted her hard drive because she had a virus and needed to reinstall her operating system, and there might not have been any incriminating evidence on the computer after all.
The standard for criminal cases is much higher, because of the recognized higher stakes. Lawyers and judges alike tread very carefully in such cases when it comes to handling evidence, because they want to avoid a mistrial or a situation in which valid and useful evidence is excluded from a trial on a technicality. In the United States, the Fifth Amendment also plays a role in considering adverse interference, as people are barred from incriminating themselves under the Fifth Amendment.