A paper in Monday's issue of PNAS reports that sleep-deprived people are up to 4.5 times more likely to sign a false confession. There’s an important weakness in the experiment, however, in that participants didn’t face any penalty for signing the confession. But the study does tie in with other evidence suggesting that specific interrogation practices can lead to false confessions, so it may be an important chunk of pixels in an emerging picture.

As it stands, there's evidence that sleep deprivation interferes with people's ability to make rational decisions. There's also evidence that most false confessions are signed after interrogations that lasted more than 12 hours. Taken together, these findings suggest that sleep deprivation could play a role in how an interrogation turns out but doesn't tell us anything about whether this does happen.

The PNAS paper suggests that sleepier people may be more likely to falsely confess but that people's individual characteristics also play a role: people who show a more impulsive decision-making approach are more likely to sign.

False declaration

To test how grogginess influences confessions, the researchers had to create a situation where participants could be accused of something they hadn't done. They had 88 participants attend two experiment sessions, seven days apart. During both sessions, which involved a series of computer-based tasks, the participants were given several dire warnings that at no point should they press the “Escape” key on the keyboard, because it would cause a loss of important experiment data. This key was chosen because it couldn’t easily be pressed accidentally.

On the night after the second session, participants either slept in a lab bedroom for eight hours or were kept up all night. The next day before leaving, they were told that they had pressed the “Escape” key during their first session seven days previously and asked to sign a statement admitting this. If they refused on the first request, they were asked one more time.

The sleep-deprived people were way more likely to sign it than the rested people: 50 percent signed on the first request, compared to only 18 percent of the rested people (which is still pretty high; nearly one in five rested people signing a false confession is worrying on its own). Then, for those who refused initially, a further 18 percent of the sleepy people signed, bringing the total figure up to nearly 70 percent. Comparatively, 38 percent of the rested people ended up signing after both requests.

The thing is, signing the declaration didn’t have any consequences, at least not beyond making people feel bad about the experiment. The researchers admit that this is a problem, but they argue that creating an experiment where signing a false declaration did have consequences wouldn't be ethical. Without having to face anything other than embarrassment, though, it’s easy to understand why so many sleep-deprived people didn’t give it much thought.

In fact, people who rated their sleepiness as higher were more likely to sign the statement. That could mean that the more sleep-deprived someone feels, the more impaired their judgment is (as the researchers suggest). Or, it could mean that when someone is super-exhausted, their judgment about the long-term consequences of signing is fine; they just care a lot less about the personal embarrassment of some researcher thinking they deleted some experiment data.

It's possible that the lack of consequences inflated a real-world effect. That is, sleep deprivation might lead to false confessions even when there are actual consequences to be faced, but not at this very high rate. But it's difficult to know for sure, as the experiment couldn't show how sleepy people balance the long-term need to avoid harsh consequences with the short-term need to sleep.

Not wrong, just not complete

It’s important to realize that, even if this study doesn’t definitely generalize to the real world, it could still be true that sleep deprivation does lead to more false confessions—it’s just that we need more evidence than this experiment. Since other evidence points in the same direction, it's possible that future experiments with different designs could give a more convincing snapshot of this effect in action.

The policy recommendations that the authors make are reasonable. They suggest recording interrogations so that the value of the resulting confession can be assessed by everyone in the legal process. Taking a few seconds to get the suspect to rate their sleepiness could also make it clear when a confession has been signed by a sleep-deprived suspect.

The difficult thing here is finding strong evidence for what does seem to be a sensible suggestion. Just gathering data from the field might show that as sleepiness goes up, so do false confessions. But it would be difficult to back up that correlation with experimental data—in the lab, it’s difficult to create ethical negative consequences for a lab-based “confession.” Hopefully this is a problem that some clever experimental design will be able to solve.

PNAS, 2015. DOI: 10.1073/pnas.1521518113 (About DOIs).