The research found that eyewitnesses to a crime who sleep before being given a police lineup are much less likely to pick an innocent person out of a lineup, at least when the guilty person is not in the lineup.
This study, published in PLOS ONE, is the first scientific investigation into how sleep affects eyewitness memory of a crime.
“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 Michelle Stepan, lead author of the study and a doctoral student in psychology.
“Putting someone in jail is a big decision based on somebody’s memory of a crime,” she added.
The team conducted an experiment in which about 200 participants watched a video of a crime where a man planted a bomb on a rooftop. After 12 hours, they then viewed one of the two computer lineups of six similar-looking people where one lineup included the perpetrator, while the other did not.
Some participants watched the crime video in the morning and viewed a lineup that night, with no sleep in between. Others watched the crime video at night and viewed a lineup the next morning, after sleeping.
In the lineup where the perpetrator was not included, participants who had slept identified an innocent person 42 percent of the time, while those who had not slept picked an innocent person 66 percent of the time.
Results of the study showed that individuals were less likely to choose an innocent suspect after a period of sleep when the perpetrator is absent from the lineup. This is the “most interesting” finding of the study according to Kimberly Fenn, an author of the study and an associate professor of psychology.
She added that this finding is relevant because false convictions too often stem from an incorrect eyewitness identification of a suspect who did not commit the crime.
On the other hand, when the perpetrator was included, both the sleep and no-sleep groups correctly identified the perpetrator about 50 percent of the time.
The results could reflect both changes in memory strength and decision-making strategies after sleep.
“In other words, sleep may not help you get the right guy, but it may help you keep an innocent individual out of jail,” explained Fenn, who is also the director of MSU’s Sleep and Learning Lab. (Related: Sleep Meditation Can Significantly Improve Rest.)
According to the National Registry of Exonerations, a project of the University of Michigan Law School, there were 149 people in 2015 who were cleared for crimes they did not commit. They had served time in prison for an average of 14 and a half years. In the United States, about 70 percent of wrongful convictions are related to false eyewitness accounts.
As believed by the researchers, the participants who slept were more likely to use an “absolute strategy,” wherein they compared each person in the lineup to their memory of the suspect. On the other hand, the participants who had no sleep were more likely to use a “relative strategy,” wherein the people in the lineup were compared to each other to determine who most resembled the guilty man relative to the others.
Stepan believed that the relative strategy increased false identifications relative to an absolute strategy in perpetrator-absent lineups.
“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 perpetrator is not in the lineup,” she said.
The team continues to further explore how sleep may directly or indirectly affect eyewitness memory.
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