A good night’s sleep makes witnesses more reliable

There are all kinds of problems with our criminal justice system, but pretty much all of them can be traced back to one root cause: squishy, infallible humans. Eyewitness testimony is one of the worst offenders, with juries seriously inclined to trust witness’ words, despite the known weaknesses of human psychology.

It’s far from a magic bullet, but researchers from Michigan State University have found that sleep can really improve the reliability of witnesses assisting with police lineups. While researchers didn’t see any difference in the number of people picking out the correct person in a lineup, they did see a marked improvement in not picking out an innocent by mistake. That may sound like a semantic detail, but it’s significant in a world where 70% of wrongful convictions come down to a false eyewitness account.

“It’s concerning that more people aren’t making the correct decision during lineups; this suggests our memories are not super-accurate and that’s a problem when you’re dealing with the consequences of the criminal justice system,” said the study’s lead author Michelle Stepan. “Putting someone in jail is a big decision based on somebody’s memory of a crime.”

To test whether sleep did indeed impact a person’s identification skills, Stepan and associate professor of psychology Kimberly Fenn had 198 participants watch video footage of a crime before they were asked to pick out the perpetrator from a lineup of six 12 hours later. There were two differences in the circumstances: first, in half the lineups, the perpetrator wasn’t actually present; and second, the time of day was shifted for a proportion of participants. In other words, while some witnessed the crime in the morning and then made their pick in the evening, others witnessed the crime in the evening, had a full night’s sleep and then made their pick the next morning.

The difference was stark. When the perpetrator wasn’t in the lineup, those who made their pick the same day got the wrong man 66% of the time, while those who slept on it fell into the same trap in 42% of instances. However, when the actual perpetrator was in the lineup, there was no difference, with both groups of eyewitnesses picking out the right man half the time.

“In other words, sleep may not help you get the right guy, but it may help you keep an innocent individual out of jail,” said Fenn.

The researchers believe that the two groups were using different ways of processing the lineup before them, with those having slept more likely to use an “absolute strategy” where they compare the suspects to their memory. Those who hadn’t slept were more likely to use a “relative” model, where they merely compare those in the lineup to each other.

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Unsurprisingly, the latter is more likely to lead to false identifications. “These findings tell us that sleep likely impacts memory processes but that it might also impact how people search through a lineup, and those search strategies might be a critical factor when the perpetrator is not in the lineup,” said Stepan.

Overlooking the fact that witnesses may struggle to sleep after witnessing a crime, if those figures sound pretty ropey, then welcome to the slightly shaky world of courtroom psychology. The problem, as I found out when I researched that feature, is that no matter how well known the weaknesses are to professionals, evidence collected in this way is still really compelling to jurors. “Witnesses can be confident and wrong. They may not have perceived things as they actually happened,” professor Amina Memon, chair of psychology at Royal Holloway University, told me at the time. “That can vary regardless of how reliable the witness appears to come across.”

And that’s just from memory: even video is flawed, as Professor Tim Valentine from Goldsmiths University told me. “If you show people good-quality images and ask them to pick out the same person if the images were taken by different cameras, the error rate is around 30% under ideal conditions.”

A good night’s sleep won’t come close to fixing these huge problems – but if one innocent person is saved conviction by a witness sleeping on it, then it certainly feels like a small but important step in the right direction.