Why everything falls apart, not just one point
A court cannot check every word separately. It assesses whether a person can be trusted, based on whether they have been caught lying. Hence the old rule: someone who lies about one thing makes you doubt everything else.
For the defense, this is a ready-made tool. Finding one discrepancy is enough, even in a minor detail, even one explainable by nervousness, and doubt spreads through the entire testimony. A witness who lies about something unimportant usually brings down the part where they told the truth, too.
A lie about a minor detail costs more than the detail itself: it wipes out everything the person got right.
The second loss is time
An investigation follows a theory. False testimony sends the investigation down the wrong path, consuming people, requests, and weeks of work. While investigators check the wrong people, real evidence grows stale: camera footage is overwritten, witnesses forget details, and things disappear.
That is why the harm from lying is not the same as the harm from staying silent. Silence leaves a case where it is, while a lie takes it off course, and finding the way back costs far more than a path left untaken.
How this ends for innocent people
The gravest consequence is convicting an innocent person. In major reviews of cases where convictions were later overturned after genetic testing, false testimony and false accusations were among the most common causes of error.
About this many out of ten overturned convictions involved a false accusation or false testimony
The motives behind such testimony are usually ordinary: fear, a deal with the prosecution, or a desire to divert suspicion from oneself or a loved one. There is often no intention at all to harm a particular person, but the result is the same.
There is also a less-discussed side. To a court, an honest witness's mistake looks just like a lie: the person confidently describes something that never happened because memory filled in the detail. Without outside evidence, it is almost impossible to tell the two apart.
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Sources
- The share of cases involving false accusations and false testimony among convictions overturned after DNA testing is reported by organizations that review such cases
- estimates vary by sample