In a criminal trial, tainted evidence, also referred to as evidence of taint, is evidence that was acquired by illegal means. For example, if authorities gather evidence using a wiretap without a proper warrant, the evidence will be deemed tainted. Because of Fourth Amendment implications, tainted evidence is generally inadmissible, and convictions supported solely by such evidence will be reversed.
There are, however, exceptions to this principle. A conviction may still be upheld even where tainted evidence was presented under several circumstances. First, in some jurisdictions, like New York, if the tainted evidence was obtained from an independent source, as opposed to an illegal search, then it may be admitted like normal evidence. For example, evidence that would be deemed tainted if confiscated by illegal search will not be tainted if found after it is abandoned by the defendant.
Second, a conviction found on tainted evidence may be sustained where it can be shown that the evidence would have inevitably been found by lawful activity even if no illegal search had taken place. This exception has been referred to as the "inevitable-discovery rule." For example, the Supreme Court has held that where self-incriminating statements to first-degree murder was illegally obtained by police, the resulting conviction could nonetheless be upheld on the basis that evidence of the illegal activity would have been discovered in the course of ordinary events.
Third, a conviction found on tainted evidence may be sustained where it can be shown that the tainted evidence was "harmless," or in other words, the defendant would have been convicted regardless of the admission of the tainted evidence. The Supreme Court has held that whether evidence is harmless depends on multiple factors, including the importance of the evidence to the prosecution's case, whether the evidence was cumulative, and the overall strength of the prosecution's case. Determinations made under this standard are highly fact dependent. For example, the Supreme Court of Virginia has found that, in a trial for abduction, the resulting conviction could be upheld even though tainted evidence was presented, because regardless of the evidence, all other fairly admitted evidence would have been sufficient to satisfy the requirements for the conviction.
Lastly, the Supreme Court has held that tainted evidence may still be used for the limited purpose of impeaching a testifying criminal defendant's credibility.
[Last updated in September of 2021 by the Wex Definitions Team]