Cooperating witnesses, incentives, and jurors’ evaluation of testimony

Someone in jail says the accused confessed. Why should we believe that person? Before we treat the reported confession as proof, we should ask what the witness gets out of telling this story. That question will drive the discussion.

First, keep the two kinds of cooperating witnesses straight. An accomplice can describe a crime in which both people were allegedly involved. Picture a drug dealer testifying against the person who supplied the drugs. A jailhouse informant might instead report something learned while incarcerated, such as another prisoner's supposed confession. These witnesses can supply useful information, but access to information doesn't guarantee honesty.

The second paragraph gives us a reason to worry: a witness may receive a reduced sentence or another benefit. Now there is a reason to fabricate helpful testimony. And the study says lying informants are rarely prosecuted. Put those facts together. The witness can gain a lighter sentence by helping the prosecution, while the risk of punishment for lying is low. We don't need to assume every informant lies to recognize that this arrangement can reward false testimony.

The courts recognize the danger. Their answer is that cross-examination can expose problems and jurors can consider the witness's motives. Those sound like useful protections. But can the jury evaluate an incentive it doesn't know exists?

A prosecutor need not always make an explicit promise. An implied offer can still give a witness reason to expect a benefit, yet that exchange need not be disclosed to the jury. Think of the difference between an announced deal and a wink suggesting that cooperation will help. The witness may respond to either one. If jurors hear only the testimony, however, they may miss the reason to doubt it. That's the first problem with relying on the safeguards.

The fourth paragraph adds a second problem: even knowing about an incentive may not be enough. The research here concerns confessions given directly by defendants, rather than confessions reported by cooperating witnesses. Jurors give those confessions too much weight even when they know the defendant was offered a benefit. The author uses that finding to raise a concern about cooperating witnesses: if jurors underestimate incentives in one setting, they may do so in the other.

Why might an innocent person confess? Imagine facing a real risk of a ten-year sentence at trial but being offered five years for confessing. A person could choose the smaller penalty to avoid the larger risk, even without having committed the crime. A threat of harsher punishment and a promise of leniency can both change what it seems sensible to say. The point isn't that every confession is false. It's that an incentive gives us another possible explanation besides guilt.

The final paragraph explains why jurors may overlook that possibility. We tend to explain behavior through the person's internal attitudes or character instead of the situation. A confession becomes evidence of guilt because we assume that only a guilty person would confess. That shortcut leaves out the pressure or reward that could also explain the decision.

Apply the same thinking to an informant. Is the witness speaking out of remorse, to make amends for wrongdoing? Or is the witness trying to get a shorter sentence? Jurors may assume the first explanation without adequately considering the second. That is why the psychological research matters to the original problem. The courts depend on testimony that incentives can distort, and their safeguards may fail either because jurors never learn about the incentives or because they underestimate incentives they do know about.